add Facebook as an elsewhere

Closes #30.

#### TODO

 - [x] Compute display_name from both first and last name.
 - [x] What's up with avatar? We have one? Where is that coming from?
 - [x] Add Facebook icon to icon font (used on homepage button and sign-in dropdown).
 - [x] What's up with `#_=_` appended to URL after login with fb?
 - [x] Get https://github.com/idan/oauthlib/pull/268 merged and released and upgraded here, or monkey patch.
Okay, `x_avatar_url` for facebook. Just saw that.