Use custom data model for storing notifications

[mathjazz]

We need to store additional data for handling notifications (e.g categories).

There's a couple ways to achieve that:

1. Use existing django-notifications library, which supports creating custom data models. We've hit some issues with this approach, see below.
2. Create our own data model and drop django-notifications. That would also allow for other optimizations. For example, the current model creates a separate instance of the Notification model for each user, including all the content and metadata.

This is a past attempt to override the existing model from django-notifications:

diff --git a/pontoon/base/models/__init__.py b/pontoon/base/models/__init__.py
index f095bdf61..4f6012455 100644
--- a/pontoon/base/models/__init__.py
+++ b/pontoon/base/models/__init__.py
@@ -4,6 +4,7 @@ from pontoon.base.models.comment import Comment
 from pontoon.base.models.entity import Entity, get_word_count
 from pontoon.base.models.external_resource import ExternalResource
 from pontoon.base.models.locale import Locale, LocaleCodeHistory, validate_cldr
+from pontoon.base.models.notification import Notification
 from pontoon.base.models.permission_changelog import PermissionChangelog
 from pontoon.base.models.project import Priority, Project, ProjectSlugHistory
 from pontoon.base.models.project_locale import ProjectLocale
@@ -25,6 +26,7 @@ __all__ = [
     "ExternalResource",
     "Locale",
     "LocaleCodeHistory",
+    "Notification",
     "PermissionChangelog",
     "Priority",
     "Project",
diff --git a/pontoon/base/models/notification.py b/pontoon/base/models/notification.py
new file mode 100644
index 000000000..cff0a7bac
--- /dev/null
+++ b/pontoon/base/models/notification.py
@@ -0,0 +1,21 @@
+from django.db import models
+from notifications.base.models import AbstractNotification
+
+
+class Notification(AbstractNotification):
+    class Categories(models.TextChoices):
+        NEW_STRINGS = "new_strings", "new_strings"
+        TARGET_DATES = "target_dates", "target_dates"
+        NEW_COMMENTS = "new_comments", "new_comments"
+        NEW_SUGGESTIONS = "new_suggestions", "new_suggestions"
+        NEW_REVIEWS = "new_reviews", "new_reviews"
+        NEW_CONTRIBUTORS = "new_contributors", "new_contributors"
+        MESSAGING_CENTER = "messaging_center", "messaging_center"
+
+    category = models.CharField(
+        max_length=20,
+        choices=Categories.choices,
+    )
+
+    class Meta(AbstractNotification.Meta):
+        abstract = False
diff --git a/pontoon/settings/base.py b/pontoon/settings/base.py
index f09bcf729..d553cf74a 100644
--- a/pontoon/settings/base.py
+++ b/pontoon/settings/base.py
@@ -1102,6 +1102,7 @@ SOCIALACCOUNT_PROVIDERS = {
 }

 # Configuration of `django-notifications-hq` app
+NOTIFICATIONS_NOTIFICATION_MODEL = "pontoon.base.models.Notification"
 DJANGO_NOTIFICATIONS_CONFIG = {
     # Attach extra arguments passed to notify.send(...) to the .data attribute
     # of the Notification object.

Which failed with the following error:

pontoon@d0168d836c37:/app$ ./manage.py showmigrations
SystemCheckError: System check identified some issues:

ERRORS:
base.Notification.action_object_content_type: (fields.E304) Reverse accessor 'ContentType.notify_action_object' for 'base.Notification.action_object_content_type' clashes with reverse accessor for 'notifications.Notification.action_object_content_type'.
	HINT: Add or change a related_name argument to the definition for 'base.Notification.action_object_content_type' or 'notifications.Notification.action_object_content_type'.
base.Notification.action_object_content_type: (fields.E305) Reverse query name for 'base.Notification.action_object_content_type' clashes with reverse query name for 'notifications.Notification.action_object_content_type'.
	HINT: Add or change a related_name argument to the definition for 'base.Notification.action_object_content_type' or 'notifications.Notification.action_object_content_type'.
base.Notification.actor_content_type: (fields.E304) Reverse accessor 'ContentType.notify_actor' for 'base.Notification.actor_content_type' clashes with reverse accessor for 'notifications.Notification.actor_content_type'.
	HINT: Add or change a related_name argument to the definition for 'base.Notification.actor_content_type' or 'notifications.Notification.actor_content_type'.
base.Notification.actor_content_type: (fields.E305) Reverse query name for 'base.Notification.actor_content_type' clashes with reverse query name for 'notifications.Notification.actor_content_type'.
	HINT: Add or change a related_name argument to the definition for 'base.Notification.actor_content_type' or 'notifications.Notification.actor_content_type'.
base.Notification.recipient: (fields.E304) Reverse accessor 'User.notifications' for 'base.Notification.recipient' clashes with reverse accessor for 'notifications.Notification.recipient'.
	HINT: Add or change a related_name argument to the definition for 'base.Notification.recipient' or 'notifications.Notification.recipient'.
base.Notification.recipient: (fields.E305) Reverse query name for 'base.Notification.recipient' clashes with reverse query name for 'notifications.Notification.recipient'.
	HINT: Add or change a related_name argument to the definition for 'base.Notification.recipient' or 'notifications.Notification.recipient'.
base.Notification.target_content_type: (fields.E304) Reverse accessor 'ContentType.notify_target' for 'base.Notification.target_content_type' clashes with reverse accessor for 'notifications.Notification.target_content_type'.
	HINT: Add or change a related_name argument to the definition for 'base.Notification.target_content_type' or 'notifications.Notification.target_content_type'.
base.Notification.target_content_type: (fields.E305) Reverse query name for 'base.Notification.target_content_type' clashes with reverse query name for 'notifications.Notification.target_content_type'.
	HINT: Add or change a related_name argument to the definition for 'base.Notification.target_content_type' or 'notifications.Notification.target_content_type'.
notifications.Notification.action_object_content_type: (fields.E304) Reverse accessor 'ContentType.notify_action_object' for 'notifications.Notification.action_object_content_type' clashes with reverse accessor for 'base.Notification.action_object_content_type'.
	HINT: Add or change a related_name argument to the definition for 'notifications.Notification.action_object_content_type' or 'base.Notification.action_object_content_type'.
notifications.Notification.action_object_content_type: (fields.E305) Reverse query name for 'notifications.Notification.action_object_content_type' clashes with reverse query name for 'base.Notification.action_object_content_type'.
	HINT: Add or change a related_name argument to the definition for 'notifications.Notification.action_object_content_type' or 'base.Notification.action_object_content_type'.
notifications.Notification.actor_content_type: (fields.E304) Reverse accessor 'ContentType.notify_actor' for 'notifications.Notification.actor_content_type' clashes with reverse accessor for 'base.Notification.actor_content_type'.
	HINT: Add or change a related_name argument to the definition for 'notifications.Notification.actor_content_type' or 'base.Notification.actor_content_type'.
notifications.Notification.actor_content_type: (fields.E305) Reverse query name for 'notifications.Notification.actor_content_type' clashes with reverse query name for 'base.Notification.actor_content_type'.
	HINT: Add or change a related_name argument to the definition for 'notifications.Notification.actor_content_type' or 'base.Notification.actor_content_type'.
notifications.Notification.recipient: (fields.E304) Reverse accessor 'User.notifications' for 'notifications.Notification.recipient' clashes with reverse accessor for 'base.Notification.recipient'.
	HINT: Add or change a related_name argument to the definition for 'notifications.Notification.recipient' or 'base.Notification.recipient'.
notifications.Notification.recipient: (fields.E305) Reverse query name for 'notifications.Notification.recipient' clashes with reverse query name for 'base.Notification.recipient'.
	HINT: Add or change a related_name argument to the definition for 'notifications.Notification.recipient' or 'base.Notification.recipient'.
notifications.Notification.target_content_type: (fields.E304) Reverse accessor 'ContentType.notify_target' for 'notifications.Notification.target_content_type' clashes with reverse accessor for 'base.Notification.target_content_type'.
	HINT: Add or change a related_name argument to the definition for 'notifications.Notification.target_content_type' or 'base.Notification.target_content_type'.
notifications.Notification.target_content_type: (fields.E305) Reverse query name for 'notifications.Notification.target_content_type' clashes with reverse query name for 'base.Notification.target_content_type'.
	HINT: Add or change a related_name argument to the definition for 'notifications.Notification.target_content_type' or 'base.Notification.target_content_type'.
pontoon@d0168d836c37:/app$