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
I came to the same conclusion when I was first adding Segment.io. I test it manually when I change things, so it would make sense for it to be part of the manual test plan.