SocialAuthBackend only needs to authenticated

[garrypolley]

In attempting to get this library to work with a permissions backend it kept failing because SociaAuthBackend was inheriting from the django.contrib.auth.backends.ModelBackend. The default Django AuthBackend assumes SQL. Since this application does both SQL and Mongo it should not make the same assumptions.

The SocialAuthBackend only needs to implement authenticate, since it only does authentication. Since it defines authenticate as a method, it's fine to inherit from object rather than the django model auth backend.

I've tested this change against a few backends for SQL and Mongo and not had any issues. Since tests are manual though, I recommend doing some as well.

for reference

https://docs.djangoproject.com/en/dev/topics/auth/#authentication-backends

[omab]

Thanks

[garrypolley]

Thank you for the quick pull request merge.