Reinstate Facebook Support

[rhodges]

Using Python Social Auth and Social Auth Django:
https://python-social-auth.readthedocs.io/en/latest/configuration/django.html

Current backends (5/8/2019):
In [2]: settings.AUTHENTICATION_BACKENDS
Out[2]:
('social.backends.google.GoogleOAuth2',
'social.backends.facebook.FacebookOAuth2',
'social.backends.twitter.TwitterOAuth',
'django.contrib.auth.backends.ModelBackend')