Rework SAMLUri to reference owning descriptor

Still allows for SAMLUri that are not owned by a descriptor as this is
not enforced at the DB level (not should it be). However for URI that
ulimately should be 'owned' by IdP/SP etc this reference allows us to
navigate back to the parent.
Add SAML IDPSSODescriptor metadata model

Makes associated changes to models and tables which IdP is responsible
for.