Use Django Reversion as an upgrade over Django Permanent

[pradumangoyal]

Django Permanent used to work for us. It provided us with removed and masked our querysets to filter out deleted objects. But it has run its course because of its use of a DateTimeField for storing whether an object has been deleted (quirky but acceptable) and its lack of integration with Django Admin.

The goal is to switch packages to Django Reversion, which offers more features, is more active and has more stars and contributors on GitHub. This would tie in with #23 because coupling with Django Admin invariably implies writing ModelAdmin classes.