Facebook auth support.

- adds Facebook as a valid 3rd-party auth provider
- moves some moderately janky JS-based logging to the server-side
- maintains integrity of current login-based logging for both old and new 3rd-party-auth flows
Mentioned this on Hipchat, but it occurred to me it'd make more sense to mention here, so:

The event "edx.bi.user.account.registered" seems unclear to me.  Is that supposed to track the first time a user signs in? or their registration?

If we're tracking first-time sign in, wouldn't it make sense to move the "alias then identify" call to immediately after a student registers? rather than when they first log in?  (I can make that change, just want to make sure it's the correct change :smiley: )