fix ImportError: cannot import name FormDefinition

[agafonovdmitry]

When DEBUG=False importing FormDefinition causes circular importing of User model.
Here is a "fix".

[ldgarcia]

@samluescher, this commit is the correct way to do it.

According to the docs:

Generally speaking, you should reference the User model with the AUTH_USER_MODEL setting in code that is executed at import time. get_user_model() only works once Django has imported all models.

See: https://docs.djangoproject.com/en/1.7/topics/auth/customizing/

I am getting the following error when running migrate, because of the get_user_model():

django.core.exceptions.AppRegistryNotReady: Models aren't loaded yet.