Peer-grading acceptance tests

This PR adds bok-choy acceptance tests for the peer-grading flow.  It covers:

1) Submitting an essay for peer grading
2) Grading calibration essays
3) Grading peer essays
4) Receiving feedback on a peer-graded essay.

It does *not* cover the staff grading UI or flagging submissions.

The setup is relatively complicated, since the peer grading workflow requires communication with the ORA service.  I have created a new stub service for ORA that simulates a simplified version of the peer-grading API.

@stephensanchez @jzoldak