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
Each call to an `analytics` method performed on the server needs to include the user's Google Analytics `clientId` in a `context` object. [See here] for an example, specifically `common/djangoapps/student/models.py`, line 767.