At the moment, when someone is mentioning a group, or using here or
all mention, we create a chat_mention record per user. What we want
instead is to have special kinds of mentions, so we can create only one
chat_mention record in such cases. This PR implements that.
Note, that such mentions will still have N related notifications, one
notification per a user. We don't expect we'll have performance
problems on the notifications side, but if at some point we do, we
should be able to solve them on the side of notifications
(notifications are handled in jobs, also some little delays with
the notifications are acceptable, so we can make sure notifications
are properly queued, and that processing of every notification is
fast enough to make delays small enough).
The preparation work for this PR was done in fbd24fa, where we make
it possible for one mention to have several related notifications.
A pretty tricky part of this PR is schema and data migration, I've explained
related details inline on the migration files.
62f423da15