Delete oldest episodes first when performing auto cleanup.

Currently, when performing an auto cleanup to make room for new episodes, AntennaPod will start deleting old episode files in alphabetic order by feed title. This behaviour is rather confusing, since old episodes from feeds that come late in the alphabet tend to hang around a lot longer than newer episodes near the beginning of the alphabet.

In my opinion, AntennaPod should delete episodes that were listened to least recently first.