When the gitlab-mentions-bot mentions someone in a note, the bot
account itself becomes subscribed to the MR and it receives emailed
update notifications (which end up being received by the releng
folks).  That's no good.  To work around this, unsubscribe from the MR
after posting a note.

Bug: T289712
Change-Id: I56ecb3c31c5141acdc6aa6245d4acc1b4a97eed4