Support Twitch.tv as an OAuth provider

Summary:
This is mostly for personal reasons / lols, but they have a perfectly functional OAuth2 API and it takes like 15 minutes to add a provider now and I was in this code anyway...

@chad, we could use JIRA, Twitter and Twitch.tv auth icons if you have a chance.

Test Plan: {F53564}

Reviewers: btrahan, chad

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D6706