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

Is this comment correct? doesn't match with the `if` condition