Reformat student submissions reports

@jbau @jrbl @dcadams 

To use, go to instructor dashboard -> Data Download -> Reports section -> Get Student Submissions Report. This instructor facing functionality does not include students with no submissions; you have this option with the management command.

Also includes management command that can be called like follows:
./manage.py lms dump_student_submissions edX/DemoX/Demo_Course -o output.csv --all-students
I think it's ok to exclude students with no submissions.

I don't think the list of students for each problem needs to always be in the same order. (Is there a use case that you can see that as needed that I haven't thought of?)  At any rate, since the problems have an order listed, the instructor could resort their spreadsheet by problem-order, student if they wanted that ordering, I think.