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 don't like how the `all_students` case modifies `answers`, but the not `all_students` case already appends `datarows`.
The intertwining logic is difficult to follow.

You can easily just have `answers` grow even in the non-`all_students` case, then add all the rows in `answers` to `datarow`s unconditionally.  That would be easier to follow.