Searching media by slug is easy on mongo. But doing the
joins in sqlalchemy is not as nice. So created a function
for doing it.
Well, and create the same function for mongo, so that it
also works.
In two cases (generating a new slug and editing the slug)
it is nice to know in advance (before the db gets angry)
that the slug is used/free. So created a db utility
function to check for this on mongo and sql:
check_media_slug_used()
In some cases (notably the mark_entry_failed function) it
is useful to have atomic update functionality on the db. On
mongo this requires special syntax.
So created an atomic_update function for mongo and started
to use it in mark_entry_failed.
