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`