Add GitHub OAuth support

[kiancm]

Requires #132 and a domain to be set up first.

Resources:

• https://python-social-auth.readthedocs.io/en/latest/backends/github.html
• https://github.com/RealmTeam/django-rest-framework-social-oauth2
• https://python-social-auth.readthedocs.io/en/latest/configuration/django.html