Add "multi answer" option to multiple choice

Getting started on this: [#47656791] Adds a boolean to the model (defaults to false) and allows authors to set it. Now to do something with it.

https://www.pivotaltracker.com/story/show/47656791
Show authors appropriate form controls for MC

If the MC allows multiple answers, the form controls should be checkboxes, not radio buttons. [#47656791] - This should finish up the author side but does not yet address the runtime/student side.

https://www.pivotaltracker.com/story/show/47656791
Removing runtime partials for MC and OR embeddables

These are handled by their _answers counterparts now.
First cut at user side of multi-answer multi-choice

[#47656837] The necessary code to show checkboxes instead of radio buttons in the runtime.

https://www.pivotaltracker.com/story/show/47656837
Refactoring MC question spec to use let
Update MultipleChoice spec to support multi-answer
Better tests for MultipleChoiceAnswer delegates
Rm harmless but confusing variables
Should accept multiple answers [#47656837]

Test to verify that the controller end already works.

https://www.pivotaltracker.com/story/show/47656837
Whitespace
Fix check box to send appropriate data [#47656837]
Update check_answer to handle multi-answers

[#47656837] This ensures the check-answer script actually sends the selected answers for checkboxes as well as radio buttons.

https://www.pivotaltracker.com/story/show/47656837
Disabling check-answer button for multi-answer MC

[#47656837] The behavior of "Check Answer" for questions with multiple answers is much more complicated than single-answer, and at the moment it's not well defined.

https://www.pivotaltracker.com/story/show/47656837