Refactor to use submissions app

[tssheth]

See https://github.com/edx/edx-submissions and the corresponding post on the edx-code mailing list: https://groups.google.com/forum/#!searchin/edx-code/submission/edx-code/YrcP2770L3I/wqGkbpN7noQJ

Using submissions will avoid the current requirement that students re-visit the SGA problem in order to have their grade recorded.

(See jazkarta#21)

[chrisrossi]

I've just pushed a first pass of this to the 'submissions' branch. It was a bit more invasive than probably any of us realized it would be. Almost all of the Python code was touched in some way or another.

We still have the requirement of accessing the StudentModule models directly in order to serialize and deserialize the the student state, since there was still a lot of student state we couldn't move into the submissions app: staff annotated file upload, instructor comments, workflow related fields.

It does achieve the objective, though, of allowing a score to be published immediately without any strange workarounds.

I still need to update tests.

[chrisrossi]

Tests are updated.

[chrisrossi]

Hi, what's the prospect for merging the 'submissions' branch? I think I was waiting for some feedback that I never got, and I see work has continued on 'master' in the meantime. This is a pretty big refactor so we don't want to let it get too far behind, unless we think we don't want to merge it.

[pdpinch]

This will be addressed in the 0.4.0 release