Internationalize "Not Graded" on front end (STUD-989)

@dmitchell @singingwolfboy an update of https://github.com/edx/edx-platform/pull/2067

(for some reason, that pull request didn't like the rebasing I was doing on it)

Anyways, I found the simplest thing was to keep passing around "Not Graded" in the backend, but display the internationalized version in the front end.