Refactor: Convert AssertionModel.Default property to AssertionModel.CreateFromConfiguration() method as per discussion in https://github.com/KentorIT/authservices/pull/69/files#r13321348
Convert the InResponseTo label to a manipulatable field that is available all the time as per discussion in https://github.com/KentorIT/authservices/pull/69/files#r13321374