When a user connects with Google OAuth and has several Google accounts, it would be nice to help them select the correct account by telling google which domain we expect the account to reside on.
Google supports an `hd` parameter under OAuth1 to select a default domain. This parameter is not documented (at least, anywhere that I can find) for OAuth2, but maybe it still works:
- Figure out if `hd` works for OAuth2.
- If it doesn't, try to find some equivalent.
- If we come up with something, let users configure it in Auth > Google.