Migrate from django-social-auth to python-social-auth

Final once-over.

Checklist:

 [ ] data.ocd.org [branch](https://github.com/opencivicdata/data.opencivicdata.org/tree/paultag/feature/sunlightauth)
 [ ] openstates.org [branch](https://github.com/sunlightlabs/openstates.org/tree/paultag/feature/psa-socialauth)
 [ ] sfcom [branch](https://github.com/sunlightlabs/sfcom/tree/paultag/feature/psa-sunlightauth)