Use collab's usermodel

[Collab](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.

This worked fine except for the issue that migrations specify the user model in foreign keys.  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.