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]
Try to fix migration in a hurry before demo.
(more hallmark commits before a demo)
Change ANSWER_IS_FINAL text to "Your answer is now locked"
Address inconsistent truthiness in hidden boolean input fields.
Move modal_edit to its own scss file.
Put options for question below the last rich text area. [#69960404]

https://www.pivotaltracker.com/story/show/69960404
Put options for question below the last rich text area. For image questions.

https://www.pivotaltracker.com/story/show/69960404
Prediction drawing questions in runtime.  [#69960232]

https://www.pivotaltracker.com/story/show/69960232
Include disabled button styling (copied from MW styles)  [#69960232]

https://www.pivotaltracker.com/story/show/69960232
prevent forward navigation with unsanswerd prediction on page [#69960454]

https://www.pivotaltracker.com/story/show/69960454
Fix tests, fix menu navigation (left side) [#69960454]

https://www.pivotaltracker.com/story/show/69960454
M-Choice Predictions Work without "Check answers?" [#70795320]

https://www.pivotaltracker.com/story/show/70795320