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
has_answer is in case all the modules have invalid state, so no rows for that problem would be written, and therefore the order should not be incremented, since nothing has been added to the csv output.