added course_complete status organizations/users api

@mattdrayer  @martynjames @markoivanovic added `course_completed` flag to the user attributes returned by organizations/users api to support MCKIN-2852. You can call it with additional parameter `course_id` and `include_complete_status` to get user's completion status like this `/api/server/organizations/1/users/?include_complete_status=True&course_id=MITx/999/Robot_Super_Course`
This is sweet! We are already pulling grade from a different API call, so ideally we'd also get ```grade``` and (for completeness) ```proforma_grade``` with the same results... perhaps then the switch is really "```include_grades```" instead of "```include_complete_status```". This also avoids the confusion between completeness (meaning the student went through the whole course) and "grade_complete" (meaning the student provided answers to all of the _graded_ work)