Create and save user object to client session after login
After the authentication process has completed (in the oauthCallback) we get the users data from Google.
Inside this data is an ID (used by Google) which we can also use as the userid for our app. We should make a new user object, fill it with the data from Google (right now it is ONLY the id) and save it to the client session as req.session["user"].
This can replace saving the entire data object as req.session["userdata"]