This bug was introduced by f66007ec83.
In PostJobsEnqueuer we previously did not fire the after_post_create
event and after_topic_create event for private message topics. This was
changed in the above commit in order to publish message bus messages
for topic tracking state updates. Unfortunately this caused the
NotifyMailingListSubscribers job to be enqueued for all posts including
private messages, and admins and the users involved in the PMs got
emailed the contents of the PMs if they had mailing list mode enabled.
Luckily the impact of this was mitigated by a Guardian#can_see? check
for each mailing list mode user in the NotifyMailingListSubscribers job.
We never want to notify mailing list mode subscribers for private messages
so an early return has been added there, plus the logic in PostJobsEnqueuer
has been fixed, and tests have been added to that class where there were
none before.
