`last_message_sent_at` has a `NOT_NULL` constraint in the DB so it should be safe to use for sorting.
This was causing two flakeys:
```
1) UserNotifications.chat_summary with public channel email subject with regular mentions includes both channel titles when there are exactly two with unread mentions
Failure/Error: example.run
expected: "[Discourse] New message in Random 62 and Test channel"
got: "[Discourse] New message in Test channel and Random 62"
(compared using ==)
# ./plugins/chat/spec/mailers/user_notifications_spec.rb:203:in `block (6 levels) in <main>'
# ./spec/rails_helper.rb:356:in `block (2 levels) in <top (required)>'
# ./vendor/bundle/ruby/3.1.0/gems/webmock-3.18.1/lib/webmock/rspec.rb:37:in `block (2 levels) in <top (required)>'
2) UserNotifications.chat_summary with public channel email subject with regular mentions displays a count when there are more than two channels with unread mentions
Failure/Error: example.run
expected: "[Discourse] New message in Random 62 and 2 others"
got: "[Discourse] New message in Test channel 0 and 2 others"
(compared using ==)
# ./plugins/chat/spec/mailers/user_notifications_spec.rb:236:in `block (6 levels) in <main>'
# ./spec/rails_helper.rb:356:in `block (2 levels) in <top (required)>'
# ./vendor/bundle/ruby/3.1.0/gems/webmock-3.18.1/lib/webmock/rspec.rb:37:in `block (2 levels) in <top (required)>'
```
Previously, restricted category chat channel was available for all groups - even `readonly`. From now on, only user who belong to group with `create_post` or `full` permissions can access that chat channel.
Instead of passing `user` to `guardian.can_chat?`, we
can just use the inner `@user` that is part of the guardian
instance already to determine whether that user can chat,
since this is how it works for all other usages of guardian
even within chat.
This is a followup of the previous refactor where we created two new
models to handle all the dedicated logic that was present in the
`ChatChannel` model.
For the sake of consistency, `DMChannel` has been renamed to
`DirectMessageChannel` and the previous `DirectMessageChannel` model is
now named `DirectMessage`. This should help reasoning about direct
messages.
