Deleting a message with a mention doesn't clear the associated notification, confusing the mentioned user.
There are different chat notification types, but we only care about `chat_mentioned` since `chat_quoted` is associated with a post, and `chat_message` is only for push notifications.
Unfortunately, this change doesn't fix the chat bubble getting out of sync when a message gets deleted since we track unread/mentions count with an integer, making it a bit hard to manipulate. We can follow up later if we consider it necessary.
This commit introduce a new API for registering callbacks, which we'll execute when a user gets destroyed, and the `delete_posts` opt is true. The chat plugin registers one callback and queues a job to destroy every message from that user in batches.
