The all inboxes was introduced in
016efeadf6f242e04daf5ef8e18c2ca708a1392d but we decided to roll it back
for performance reasons. The main performance challenge here is that PG
has to basically loop through all the PMs that a user is allowed to view
before being able to order by `Topic#bumped_at`. The all inboxes was not
planned as part of the new/unread filter so we've decided not to tackle
the performance issue for the upcoming release.
Follow-up to 016efeadf6f242e04daf5ef8e18c2ca708a1392d
9d5da2b383