Deleting a POTD from the admin doesn't invoke Photo.delete

[brian]

When you delete items from an admin changelist, it apparently calls delete() on the queryset, which does all the deleting in the database. It does not call delete() on the individual objects in the queryset.

Because of this, our customized logic in Photo.delete() is not getting called.

[brian]

Fixed in r578. Moved the updating of the Sequence object into a signals module. Listen for post_save and post_delete.