Rough in Facebook
Add some facebook icons
Add Facebook (and Google+) to social icons
Extract display name and avatar for Facebook

Display name was easy. Avatar requires a second API call, which needs
the user_name we previously extracted. Extractors now take the already-
extracted UserInfo as the first argument.
Workaround wart from Facebook security measure
Upgrade to forked oauthlib
Deemphasize Facebook on homepage

We're just getting started.
Added facebook to list of platforms used in fake_data.py
Merge pull request #2763 from gratipay/facebook

add Facebook as an elsewhere