[#47565665] enabled facebook login

@wanxin  Would you mind trying this out?

This is slightly more complicated. We have a bunch of sensitive information (e.g. Facebook App ID, Facebook App Secret) that we cannot store on git (because git history is almost forever.) You should have received, in another email, a file containing those secrets; make sure you plant that file at the right place before you start the server.

Then just click on 'Sign in with Facebook'.

As usual, execute `bundle install` and `rake db:migrate` after pulling.