Generalize grade report to allow display of any report

We would like to be able to generate arbitrary reports and expose them to the instructor by simply uploading them to S3.  The existing grade download infrastructure pretty much supported that already, however, all of the internal and external structures were referring to the reports as exclusively grading related.

Fixes: AN-590

lms/djangoapps/instructor/views/api.py

@@ -33,7 +33,7 @@ from student.models import unique_id_for_user
 import instructor_task.api
 from instructor_task.api_helper import AlreadyRunningError
 from instructor_task.views import get_task_completion_info
-from instructor_task.models import GradesStore
+from instructor_task.models import ReportStore
 import instructor.enrollment as enrollment
 from instructor.enrollment import enroll_email, unenroll_email, get_email_params
 from instructor.access import list_with_level, allow_access, revoke_access, update_forum_role
@@ -770,16 +770,16 @@ def list_instructor_tasks(request, course_id):
 @ensure_csrf_cookie
 @cache_control(no_cache=True, no_store=True, must_revalidate=True)
 @require_level('staff')
-def list_grade_downloads(_request, course_id):
+def list_report_downloads(_request, course_id):
     """
     List grade CSV files that are available for download for this course.
     """
-    grades_store = GradesStore.from_config()
+    report_store = ReportStore.from_config()
 
     response_payload = {
         'downloads': [
             dict(name=name, url=url, link='<a href="{}">{}</a>'.format(url, name))
-            for name, url in grades_store.links_for(course_id)
+            for name, url in report_store.links_for(course_id)
         ]
     }
     return JsonResponse(response_payload)