Use collab's user model

[Collab](http://www.github.com/cfpb/collab) is migrating to a custom user model since the default 30 characters isn't enough for the username.

Phase 1 - The original 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).

This worked fine except for the issue that [migrations specify the user model in foreign keys](https://github.com/cfpb/idea-box/pull/72/files#diff-8f577bf031726094c5ae771052f0bf56R13).  This would cause issues when migrating a system that is not running as a collab child app.

Phase 2 - the best solution we could find to continue to allow ideabox to run as an independent app was to import collab as a library.  Ideabox does not inherit collab styles, pages, or models with the exception of the core.CollabUser model.  This required some adjustments to the buildout environment to include collab.