Support prompt=none workflow

Hydra does not currently support the prompt=none authentication request but we can work around this by introducing a new scope, e.g. prompt:none.

If a consent request has this scope, the consent app should determine if the user is currently logged in and, if so, grant the request. If not, the request should be rejected without showing the Raven login screen. This allows us to implement transparent login and token refresh for frontend public clients like the IAR.