Ensure Answer associated with Question #4084

When creating or updating a Response validate that the
Answer being set is one of the Answers associated with the Question.

This probably should be added to Surveyor as a validation but is included
here to address the #4084 issue going forward with Cases. If Surveyor is
ever changed to add this validation we should remove it here
(see TODO note above validate callback)
Raise error #4084

instead of using ActiveRecord validations
to ensure data integrity
Merge pull request #174 from NUBIC/4084-ensure_answer_for_response_associated_with_question

Ensure Answer associated with Question #4084