Feature: allow external users

(This is not so new feature, as we had it in the Tornado version, but brought back in the Flask version too)

You can specify allowed external users (=users not from your Google Apps domain), which will be allowed to sign in too. If no external users specified it will use federated login with your domain (so users already signed in with your Google Apps domain won't need to choose which account to use).

Also if no Google Apps domain specified, *it will let any users to sign in* (used in the demo re:dash instance).