Add assignment 2 features

There's a test instance of this running at alopezfoo.appspot.com. Major difficulty was figuring out how to get memory use down to reasonable levels. I can't tell if it will eat up all our free resources after a few uploads, but would like to put it out and see what happens. In a pinch, we can still grade output sent over email; it is a complete information assignment.

Includes many frontend/ backend changes:
-Submission history
-Upload status bar
-Better memory use in decoding assignment scorer
-Distributed decoding assignment scoring
-Oracle computation for decoding assignment