Implement support for Oauth2 based login integrations

Implement support for Oauth2 based login integrations

This creates Google, Github, Microsoft and Facebook login integrations.
Other providers can also be added if needed. Accounts still need to be
created in the community auth system, and will be automatically created
on first login, when the user also gets to pick a username. Once an
account exists, it will be matched on email address from the external
systems.

No methods are enabled by default, as they all require encryption keys
and identities configured in local_settings.py.

Review by Stephen Frost, Jonathan Katz and Daniel Gustafsson.