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
If the user has just registered, we need to alias the user's anonymous history with the user's new identity, then map the user's activity to their newly assigned ID, all before tracking the creation of their account. [See here](https://segment.io/docs/integrations/mixpanel/#tracking-exclusively-server-side) for Segment.io's suggestions on this. I'd recommend a pattern like the following:

```
if user has just registered
    analytics.alias(anon_id, user_id)
    analytics.identify
    analytics.track("edx.bi.user.account.registered")
else
    analytics.identify
    analytics.track("edx.bi.user.account.authenticated")
```

The functions `assessUserSignIn`, `trackAccountRegistration`, and `trackReturningUserSignIn` in `common/static/js/src/utility.js` formerly implemented this logic.