R1:1620bce842ea

Add Google as an OAuth2 provider (BETA)

Summary:
This is pretty straightforward, except:

- We need to request read/write access to the address book to get the account
ID (which we MUST have) and real name, email and account name (which we'd like
to have). This is way more access than we should need, but there's apparently no
"get_loggedin_user_basic_information" type of call in the Google API suite (or,
at least, I couldn't find one).
- We can't get the profile picture or profile URI since there's no…