Look for response.authResponse in FB.login when a user is successfully logged in

According to https://developers.facebook.com/docs/reference/javascript/FB.login/, we should be looking for response.authResponse.

Tested by popping up the auth dialog and hitting cancel. Prior to fix, the page would say "Loading profile information..." and then redirect the user to a full page auth dialog. After fix, the user gets a message saying that they could not be logged in.