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
A "first-time" login should be the first time Bob logs in *ever*, since it's used as the condition on which to fire the `edx.bi.user.account.registered` event.