Model changes for prediction questions and answers

(TODO in answers, rename is_prediction to is_final)

Starts: [#69960404] [#69959708] [#69960232]

https://www.pivotaltracker.com/story/show/69960404
https://www.pivotaltracker.com/story/show/69959708
https://www.pivotaltracker.com/story/show/69960232