Diana/fix open ended input

Altered staff grading to display message about ml grading performance.

- also updated some out-of-date docs

Vik/modify open ended

- set default queue in response type--don't use the default per-course one

- can't change the xqueue one because there's data that uses it
- we checked, and no one has sent out survey links that use the old sha1() format

doing this because we'll need the LMS to be able to send the anon student id to backend services e.g. for peer grading, and passing two different anon student ids to xmodule (one for xqueue, one for other uses) is just asking for confusion.

- specify mime type
- right urls

- now getting requests from js to server and back, with mocked service.

- look up test courses by id, not name