add 'Submission History' to proctor admin panel

Staff users can now navigate to a problem, click 'Proctor Submission
History' button in the proctor admin panel, type a username and view
the user's attempts for each problem they've attempted within the
problem bank.
add submission_history management command

This shows the entire submission history for every problem in a course
for a given user. Takes a course id and student username as input.