Wescott master exercise work

@jbau 

Fixes for #1554, #1783, #1813 

"Hide Answers" button is disabled by default and enabled when "Check Answers" has been clicked.

First question in single question set has "Previous Question" button disabled, becomes enabled as user progresses. Last question in set has disabled "Next Question" button.

JS confirm dialog pops up when user clicks "Submit" if any questions left blank. Still can submit, but have to click "OK". If all questions in the set have an answer, no confirm dialog, it just submits.