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

How about this?
```
if (subQuestionIndexAndChoice.length != 2) {
    continue;
}
```