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
@rlucioni Do you know if there's a way to query the analytics output?  I'm sure we do some kind of event tracking on student registrations...  (This might be a question for the analytics team, not sure)