fix rereview queue from breaking due to soft deleted addons (bug 876819)

Sitch

1. Theme is re-uploaded/updated.
2. addon.delete() (status => amo.STATUS_DELETED)
3. Theme tries to load in the theme queue.
4. RereviewQueueTheme object tries to access header/footer URLs.
5. Calling theme.addon raises Addon.DoesNotExist

Fix

RereviewQueueTheme manager that ```excludes``` soft-deleted add-ons.