Replace per-post read tracker with per-thread one

[rafalp]

This PR brings back "classic" read tracker to Misago.

This tracker actually tracks two separate read times:

• Timestamp of when user has read everything in a category
• Timestamp of latest read post in a thread

Whenever user sees new post, we create or update existing entry for a thread. We also check if there are other unread threads in this category and if there aren't, we update the category entry. This second step forums perform life, but we could delay it to a celery task to keep thread view speedy.

Fixes #956 #1585 #1618

Superseds #1449

TODO

• Get categories read status
• Get threads read status
• Get thread posts read status
• Update thread read status when posts are read
• Update category read status when posts are read
• Mark all threads as read
• Mark all private threads as read
• Mark category as read
• Mark all categories as read
• Move "Mark as read" base templates to misago/mark_as_read/...
• Update category read status if there are no unread threads on list
• Add util on user model to clear their unread_private_threads if its not zero
• Update watched thread read time
• Add "unread" filter to threads lists
• Update clearreadtracker management command
• Regenerate hooks reference

[rafalp]

There are 146 tests failing. I will fix those in future PRs.