WIP: Rough sketch at responsibility split between submissions/scores storage and workflow

Not fully baked, but I'd rather get it out sooner. I'm not really as concerned about specifics with this one (like specific fields), but whether folks think that it's reasonable to have this split in responsibilities between a workflow aware `openresponse.*` series of apps (of which `openresponse.peer` would be the first) and the workflow-ignorant `submissions` app.

I made these new apps in the `/apps` directory partly to make it easy to differentiate, and partially because that's where I'd like to see our code live. To get `setup.py` to work with it, all we need to do is specify `apps` as our `package_dir`.

Also as a note -- because XBlocks are tied to the problem definition in one sense (there's a basic mapping of tag name to XBlock class), if we're going to ever have problems that are gradable via multiple channels (e.g. self + peer), then we basically have to have one XBlock to rule them all.