LoginFlow Rework

Part of this is #166 (closed) with !255 (closed) due to the new LoginService.

General:

• Implement OIDC (!255 (closed))
• Rework SplashScreen (!256 (merged))
• Rework GreeterScreen (!257 (merged))
• Implement new Screens ( login_decision and login_information) (!258 (merged))
• Hook everything up...

After Auth-Rework

• Only show "LogOut"-Button when token present.
• Add tests for correct login-checks.
• Add OS-Side SplashScreens.