Remember ReturnUrl query parameter from SignIn page

Remember ReturnUrl query parameter from SignIn page and redirect back to that page after an AuthnResponse
I think that the `Saml2Response` class should only expose the data actually read from the message. With the current setup, the ReturnUri is grabbed from an internal data structure. I think it should be clearly visible that it wasn't part of the original message. Maybe create an `RequestState` object that is automatically set as a property on the `Saml2Response` when looked up from `PendingAuthnRequest`. `RequestState` might be as easy as renaming `PendingAuthnRequestData`.