This commit adds the messages_count column for ChatChannel messages,
which is the number of not-deleted messages in the channel.
This is not updated every time a message is created or deleted in a
channel, so it should not be displayed in the UI.
It is updated eventually via Jobs::ChatPeriodicalUpdates, which
will have additional functions in future after being introduced
here.
Also update these counts for existing channels in a post migration.
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.
Martin Brennan
Roman Rizzi