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
Model changes: rename is_prediction to is_final in answers.

[#69960404] [#69959708] [#69960232]
WIP: Started on Runtime CSS / Coffeescript for prediction.

[#69960404] -- in progress –
create 'prediction' option for open response questions.

https://www.pivotaltracker.com/story/show/69960404
Completed runtime implementation for open response prediction questions

[#69960404]
https://www.pivotaltracker.com/story/show/69960404
Changes to authoring form for Multiple Question predictions.

[69959708]
https://www.pivotaltracker.com/story/show/69959708
Runtime changes for multiple-choice answers (prediction mode)

completes: [69960232]
https://www.pivotaltracker.com/story/show/69960232
Authoring form changes for image prediction questions:

[#69960232]
https://www.pivotaltracker.com/story/show/69960232
Make the editing forms uniform. Use I18N files for text.

[#69960232]
[#69959708]
[#69960404]