Trigger Mention and QuoteMention notifications upon post update

[danneu]

Currently Mentions/QuoteMentions only create notifications upon post creation.

If a user updates a post to add mentions or fix typos in their mentions, then no new notifications will be created.

The system must track send-notifications per post so that notifications are not re-sent.

[danneu]

Implementation proposal:

• Instead of deleting notifications, give them an is_deleted boolean field. (must update UNIQUE notification indexes with WHERE is_deleted = false)
• Whenever a post is updated, extract the mentions from the markup, query for the notifications that already exist for this post, and then create notifications for any new mentions
• If a post has already created MAX_NOTIFICATION_COUNT_PER_POST quantity of notifications, then do nothing.