#59338994 Added QuestionResponse and allow them to be updated through SurveyResponse#show
#59338994 Added validation to QuestionResponse subclasses.
This required getting rid of the becomes(QuestionResponse) hack to get around STI paths, but it turns out you can explicitly specify a path to best_in_place anyway.
Also extracted custom nil (placeholder) text for specific QuestionResponse subclasses.
#59338994 Indicate unanswered questions