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
@flowerhack unfortunately we don't actually track user registrations explicitly. You can see the middleware-logged "request" events that can give some insight, however, those are emitted regardless of whether the registration is successful or not, so they aren't a particularly strong metric and will differ significantly from the database table.

If you want to see this data anyway, talk to me offline.