Fix pollution of auth_user_model

As discussed on the mailing list, Zinnia has been polluting auth_user_model through its proxy model.

https://groups.google.com/d/msg/django-blog-zinnia/yckq6CJk7A4/peN8zcxfobsJ

This patch attempts to fix the issue by using an abstract base class as suggested in the Django documentation.

https://docs.djangoproject.com/en/dev/topics/db/models/#proxy-model-managers

Because this is an obscure issue that affects models outside of Zinnia, I'm not sure what the most appropriate test case is.