Data upload

[TIM-538](https://edx-wiki.atlassian.net/browse/TIM-538)

Changes:
* Generate assessment and submission CSV data.
* Add a management command to upload archive of CSV data to S3

Notes:
* The number of database queries scales linearly with the number of submissions.  The SELECT queries all use `submission_uuid`, which should be indexed, but this could be slow for large numbers of submissions.
* I'm using database fixtures in the tests on the assumption that we'll need to be backwards compatible at the database level.  This may be a maintenance issue when we make database migrations.  The alternative would be to make API calls directly, but I think this approach better decouples the data stuff from the other API calls.
* I'm putting the tarballs in {S3_BUCKET}/{COURSE_ID}/{DATETIME}.tar.gz

@stephensanchez