[core] Accessing About App without loging does not always trigger authentication

Clear browser cookie or open a new private chrome tab, try reach the 9999:/about page of your Hue. Then click login.
It will show you the local database login instead of what has been configured. In our case, it fails to use SAML authentication. This is really confusing to users since there are no links you can click that can take you out of the about app.
Changed the sign in button to point at the home page, which will trigger the correct login mechanism.
Patch attached.