Updates check-answer.js to send multiple answers

[#50756521] The ID is now that of the answer object, and the selected choices are sent as a comma-separated list of IDs.

https://www.pivotaltracker.com/story/show/50756521
Specs for updated Check Answer [#50756521]

https://www.pivotaltracker.com/story/show/50756521
Re-enables "Check Answer" for multi-answer multi-choice

[#50756521] 

https://www.pivotaltracker.com/story/show/50756521