AI concurrency/perf tests

Initial attempt to update the performance tests for example-based assessment.  Also cleans up the peer/self perf tests:

* Load all steps in (almost) parallel using `gevent`.
* Stop checking whether the UI lets us submit/self-/peer-assess and just do it.  I've submitted separate pull requests that stop this approach from causing 500 errors (although it will return JSON with `success` set to `False`)
* Use continued grading some of the time.

I ran into some issues getting the Celery graders to finish training, so I wasn't able to verify the full test in a sandbox.  I'll confirm that it works for the full flow tomorrow, but I don't think that will require changes to the tests.

@stephensanchez