Wescott master exercise work

@jbau When you get a chance...

The highlights:
* Show/Hide Explanation button now instead of Hide Answers + Show/Hide Explanation link in Explanation area. (#1866)
* "Check Answer" instead of "Check Answers" for interactive exercises (#1865)
* "Check Answer" changes to "Checking..." while AJAX call in progress
* Per-question Show/Hide Explanation buttons on Summative exam results/report page