Use collab's user model

[Collab](www.github.com/cfpb/collab) is migrating to a custom user model since the default 30 characters isn't enough for the username.

The intent of this PR was to decouple django.contrib.auth.models.User from the code, and instead use django.contrib.auth.get_user_model() and AUTH_USER_MODEL as described in the [django documentation](https://docs.djangoproject.com/en/1.5/topics/auth/customizing/#substituting-a-custom-user-model).