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'd rather not see modification of `row` within this loop.  it's hard to picture what would be constant in the inner loop, especially `row` as defined by a previous loop iteration is still present at the start of the next iteration.  better to make data across iterations explicitly independent.