PERF: fix performance of chat email notifications

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

plugins/chat/plugin.rb

@@ -72,7 +72,6 @@ after_initialize do
     Bookmark.prepend Chat::BookmarkExtension
     User.prepend Chat::UserExtension
     Group.prepend Chat::GroupExtension
-    Jobs::UserEmail.prepend Chat::UserEmailExtension
     Plugin::Instance.prepend Chat::PluginInstanceExtension
     Jobs::ExportCsvFile.prepend Chat::MessagesExporter
     WebHook.prepend Chat::OutgoingWebHookExtension