Unread mentions aren't being reduced when a message is deleted

[madsroskar]

1. Send new messages to someone with @mentions
2. Soft (or hard) delete one of the messages
3. Call channel.watch()
4. Call channel.countUnreadMentions();

Result: Unread count decreases as expected, but unread mentions don’t.

[vanGalilea]

Hey team! Please add your planning poker estimate with Zenhub @vishalnarkhede @madsroskar @santhoshvai