Address #1554, #1783, #1813; Minor tweaks to come before pull request

*"Hide Answers" button starts off disabled, toggles based on "Check Answer"
and "Hide Answer" clicks
* "Previous Question" button starts off disabled, becomes enabled as user
progresses through set; "Next Question" button disabled when user gets
to last question
* submit-button click handler has JS confirm function, just need to actually
go through userSelections and look for empty inputs

	modified:   templates/exams/view_exam.html
Merge branch 'master' of https://github.com/Stanford-Online/class2go into wescott-master-exercise-work
Default Hide Answers button to disabled in in-video exercises

	modified:   templates/exams/videotest.js
	modified:   templates/exams/view_exam.html
Complete fix for #1783

Check that user has at least something for each unique named element
or group of elements in a form

	modified:   templates/exams/view_exam.html
Merge branch 'master' of https://github.com/Stanford-Online/class2go into wescott-master-exercise-work
fixed hardcoded 'problem set' in confirm msg
Merge pull request #1824 from Stanford-Online/wescott-master-exercise-work

Wescott master exercise work