Add student submission reports, management and dashboard functionality

@jrbl If you wouldn't mind taking a look.
I have a mild preference for the convention of using -c to specify course_id, rather than a positional parameter. I think the virtues of this approach are twofold:
- It's congruent with how most of the other management commands in -platform work
- You can put the courseid anywhere on the command-line you want, which is nice because then you can make it the last argument. This is nice because when you want to run the command for several different courses, you can edit your line and rerun with just <up><^E><^W>

These are small things though.