[To review] Twitter auth

This adds Twitter login functionality to the project. While OAuth itself was easy to implement thanks to python-social-auth, this was a real pain in the ass to achieve because of all the changes to the user model that impacted almost every part of the application (especially migrations). Anyway, besides adding Twitter auth, we also store the user Twitter avatar for future use.

Please note that you'll need to set the following 2 env variables to make it work:

* SOCIAL_AUTH_TWITTER_KEY
* SOCIAL_AUTH_TWITTER_SECRET

You can get keys by creating a dummy app on http://dev.twitter.com/.