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 Plus API

access and Google users don't have meaningful public pages otherwise.

• Google doesn't save the fact that you've authorized the app, so every time

you want to login you need to reaffirm that you want to give us silly amounts of
access. Phabricator sessions are pretty long-duration though so this shouldn't
be a major issue.

Test Plan:

• Registered, logged out, and logged in with Google.
• Registered, logged out, and logged in with Facebook / Github to make sure I

didn't break anything.

• Linked / unlinked Google accounts.

Reviewers: Makinde, jungejason, nh, tuomaspelkonen, aran

Reviewed By: aran

CC: aran, epriestley, Makinde

Differential Revision: 916