instructors: can add 'Rubric' type questions #2757

Fixes #2757 

Some other things related to rubric questions that could be done either in this issue or separately:
Instructor Results Page - Add a column for sub-question
Feedback Edit page - highlight row/column when cursor hovers over delete row/column button
InstructorSampleData.json - Add rubric question

I changed the JS behaviour on the page, to disable all questions(inputs) on page load. Previously, only when the user clicks edit on one question, do the other questions get disabled.