grading utilities: hitgrader and regrade (#2045)

- hitgrader manually runs the autograder, with an option to override the grading endpoint.  Useful to see if a grader is working.

- regrade, from the commit log:

> Walk through examrecords for a course and re-run the student-submitted
> answers to see if any scores have changed.  If so, then print a line
> with "REGRADE", and if -u is specificied, actually update the database.