From #1067, updated Google Auth docs to reflect API changes

Edited documentation to steer people toward `GoogleOAuth2Mixin` and properly register their apps with Google. This is required due to a recent API change.

Sources:
 * [Google timetable](https://developers.google.com/+/api/auth-migration#timetable)
 * [Server migration instructions](https://developers.google.com/+/web/signin/redirect-uri-flow)
 * [Relevant stackoverflow](http://stackoverflow.com/questions/23773275/changed-domain-error-openid-auth-request-contains-an-unregistered-domain)