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
can you explain the logic behind `has_answer` again?  Why do you only increment `order` when there is an answer?  When might there not be an answer?