Add fixtures for proofing the LMS

At long last: [Tim-226](https://edx-wiki.atlassian.net/browse/TIM-226)

Notes:

1) I ended up using dumpdata / loaddata instead of the management command, because we are storing the submission ID in an XBlock field.  (When I tried to change this to query the submissions API directly, I got integrity errors as each handler tried simultaneously to get-or-create a student item.)  Unfortunately, this ties the fixtures to the current database schema -- if and when we change this, we will need to (a) load the fixtures, (b) run migrations, and (c) recreate the fixtures using the script in this PR.

2) I found and resolved a number of bugs in order to get through the workflow in the LMS.  The difficult one is https://github.com/edx/XBlock/pull/193, which requires that my PR is merged into XBlock, then edx-platform is updated to include those XBlock changes.

3) I suspect there's something wrong with the JavaScript click handlers when multiple openassessment XBlocks are loaded in a section.  I noticed that sometimes submitting a response would cause a "EOMULTI" error from the server (multiple submissions); similarly, a few times the UI seemed to "forget" that I had already self-assessed.

4) I was not able to get multiple peer assessments to work.  I didn't track it too closely, but it seemed like submissions were not showing up in peer grading that should have been, so the test account never got enough submissions to get a grade.  As a compromise, the grading page in the fixture has one peer assessment, but obviously this is not ideal.

@ormsbee @stephensanchez