Adds changes to send CustomAlerts via Slack, #PG-4530

[AltamashShaikh]

Description

Adds changes to send CustomAlerts via Slack

Issue No

#PG-4530

Steps to Replicate the Issue

1. Requires Changes to enable send alerts via Slack, #PG-4530 plugin-CustomAlerts#209
2. Create a new alert and you should see Slack enabled in "Send alerts via" section.
3. Create a new alert, which can be triggered like unique_visitors < 2
4. Execute this command to run alerts ./console core:run-scheduled-tasks --force "Piwik\Plugins\CustomAlerts\Tasks.runAlertsDaily_{alertID}

Checklist

• [✔] Tested locally or on demo2/demo3?
• [✔] New test case added/updated?
• [✔] Are all newly added texts included via translation?
• [✔] Are text sanitized properly? (Eg use of v-text v/s v-html for vue)
• [NA] Version bumped?

[snake14]

Looks good and posted to the Slack channel as expected 👍

I had a couple nitpick/optional comments. My only other thoughts were:

1. It's a bit screenshot heavy. Could any of those screenshots use DOM instead of image comparison?
2. It would be good to document the methods with comment blocks, especially the public ones.

[snake14]

Thank you for the changes. Adding the comment blocks did highlight how many method parameters aren't typed, but that's probably out of scope.