Fix use of get_user_model()

Using `get_user_model()` in a `models` init is very dangerous. Because it can try to import your `User` model before it is in the AppCache. This suddenly broke my project. Replaced `user` with the model label, which always works and prevents circular dependency failures.
Merge pull request #161 from artscoop/patch-2

Fix use of get_user_model()