Add backend support for OAuth 2.0 application authentication

[AustinTSchaffer]

Forked from #37. PR #39 adds support for "auth w/ password" authentication. Adding support for "Login with GitHub" and "Login with Google" would be a nice to have. This effectively means adding support for OAuth 2.0 to the backend, so there will likely be some code sharing between these 2 auth methods.

GitHub

• https://developer.github.com/v3/guides/basics-of-authentication/
• https://github.com/settings/applications/new
• https://docs.github.com/en/free-pro-team@latest/developers/apps/authorizing-oauth-apps

Google

• https://developers.google.com/identity/sign-in/web/sign-in