A more useful admin interface

I originally tried to do something more clever with inlines, but threw that away. My main concern is that we have models that should never really be edited (only added to), and Django doesn't really accomodate that cleanly out of the box. There are a couple ways to try to kludge it, but I've opted to just make everything read-only instead. This means the admin is good for investigation, but won't really be useful for correcting data with this PR. I thought this was preferrable to risking data corruption/loss from misuse.

This currently covers submissions, scores, student items, and rubrics. I still need to do assessments. I'm putting this up now so it's visible in case someone needs to pick it up and run with it, but I'm still hopeful that I'll get time to work on it in the next couple of days.