Moved Django compatibility functions to utils.compat module
Add get_user_model_name() to prevent circular imports

Instead of linking to the User model by class, use the "auth.User"
string so `get_model()` won't be called while `fluent_pages.models` is
still loading. That causes circular imports, which happend at:

* `import fluent_pages.models`
* running `runtests.py`