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
The event `edx.bi.user.account.registered` is supposed to track the creation of a new account with edX (i.e., registration). With that in mind, I agree that the best place to put the alias-identify-track block is immediately after a student registers. It currently appears after the user's first sign-in because of the way this was done in the JS, where the user's very first sign-in after visiting the registration page was taken as an indicator of account creation.