Add Google OAuth2 authentication

Everything was already in place but the link in the registration page,
assuming the app key was set up properly in:
https://code.google.com/apis/console
That is, a key and secret were generated and copied into the server's
environment file, and the URL
<app>/complete/google-oauth2/
was added to the "Redirect URIs" there.
Fix #175 by showing the vote count when upvoting
Merge pull request #189 from danielhers/dev

Add Google OAuth2 authentication