Callback method for the Google Calendar API authorization process.
Once the user grants consent with his Google Calendar data usage, the Google
OAuth service will redirect him back in this page. Here we are going to store
the refresh token, because this is what will be used to generate access tokens
in the future.
IMPORTANT! Because it is necessary to authorize the application
using the web server flow (see official documentation of OAuth), every
Easy!Appointments installation should use its own calendar api key. So in every
api console account, the "http://path-to-e!a/google/oauth_callback" should be
included in an allowed redirect url.