Tighten scope requests with Google OAuth

Summary: We currently make a ludicrously gigantic permission request to do Google auth (read/write access to the entire address book), since I couldn't figure out how to do a more narrowly tailored request when I implemented it. @csilvers pointed me at some much more sensible APIs; we can now just ask for user ID, name, and email address.

Test Plan: Created a new account via Google Oauth. Linked/unlinked an existing account. Verified diagnostics page still works correctly. Logged in with a pre-existing Google account created with the old API (to verify user IDs are the same through both methods).

Reviewers: btrahan, vrana, csilvers, Makinde

Reviewed By: csilvers

CC: aran

Differential Revision: https://secure.phabricator.com/D2378