Facebook login

[cyberSmi7h]

Google login is good, but we need also social media profile picture + link to FB somehow.

[egonelbre]

For multiple provider support we need to also implement multiple profile linking. We already had issues with people logging in with different gmail accounts and getting confused. Adding easy support for multiple authentication providers is already planned with #60 changes.

To support multiple providers we need:

1. page for user to "login" to another provider while already logged in (effectively, linking them together)
2. page for admin to merge users to ensure people are deduplicated.
3. oauth2 credentials for facebook