Google provider doesn't persist token, socialUser is lost on browser refresh

[lwestfall]

GoogleLoginProvider should persist the token(s?) to localStorage, and try to get them on initialize so it can instantiate _socialUser when present.

The issue is that on browser refresh for a signed-in user, _socialUser will always be null unless the user signs back in (even if we persist the token outside of the provider). This defeats any possibility of refreshing the token after a refresh (which itself seems broken, see #745)

I can attempt a PR this week.

Details to work out (some are notes to self):

• How to handle expired tokens (should it not instantiate _socialUser in this case?)
• signOut should clear anything the provider persists

[lwestfall]

Dupe of #741