When chat is enabled, there's a scheduled job that runs every 5 minutes to check whether we need to send a "chat summary" email to users with unread chat messages or mentions.
On Discourse with a large number of users, the query used wasn't optimal and sometimes taking minutes. Which isn't good when the query is called every 5 minutes 😬
This PR reworks the query in `Chat::Mailer.send_unread_mentions_summary`.
Instead of starting from the `users` table, it starts from the `user_chat_channel_memberships` table which is the main piece tying everything together.
The new query is mostly similar to the previous one, with some bug fixes (like ensuring the user has `allow_private_messages` enabled for direct messages) and is also slightly simpler since it doesn't keep track of the `memberships_with_unread_messages` anymore. That part has been moved to the `user_notifications.chat_summary` email method.
The `UserEmailExtension` has been deleted since that was using to N+1 update the `user_chat_channel_memberships.last_unread_mention_when_emailed_it`(quite a mouthful 😛) but that's now done directly in the `user_notifications.chat_summary` email method.
The "plat de résistance" of that PR - the `user_notifications.chat_summary` method has been re-worked for improved performances 🚀
Instead of doing everything in one query, it does 4 tiny ones.
- One to retrieve the list of unread mentions (@something) in "category" channels
- One to retrieve the list of unread messages in "direct message" channels (aka. 1-1 and group discussions)
- One to load all the chat messages for each "category" channels from the last unread mention
- One to load all the chat messages for each "direct message" channels from the last unread message
All the specs for both `Chat::Mailer` and `UserNotification.chat_summary` have been rewriten for easier comprehension and faster execution (mostly by not using chat services which makes the specs go 10x slower...)
Internal ref - t/129848
