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/tests/test_api.py

@@ -130,7 +130,7 @@ class TestInstructorAPIDenyLevels(ModuleStoreTestCase, LoginEnrollmentTestCase):
             ('send_email', {'send_to': 'staff', 'subject': 'test', 'message': 'asdf'}),
             ('list_instructor_tasks', {}),
             ('list_background_email_tasks', {}),
-            ('list_grade_downloads', {}),
+            ('list_report_downloads', {}),
             ('calculate_grades_csv', {}),
         ]
         # Endpoints that only Instructors can access
@@ -794,9 +794,9 @@ class TestInstructorAPILevelsDataDump(ModuleStoreTestCase, LoginEnrollmentTestCa
         self.assertTrue(body.startswith('"User ID","Anonymized user ID"\n"2","42"\n'))
         self.assertTrue(body.endswith('"7","42"\n'))
 
-    def test_list_grade_downloads(self):
-        url = reverse('list_grade_downloads', kwargs={'course_id': self.course.id})
-        with patch('instructor_task.models.LocalFSGradesStore.links_for') as mock_links_for:
+    def test_list_report_downloads(self):
+        url = reverse('list_report_downloads', kwargs={'course_id': self.course.id})
+        with patch('instructor_task.models.LocalFSReportStore.links_for') as mock_links_for:
             mock_links_for.return_value = [
                 ('mock_file_name_1', 'https://1.mock.url'),
                 ('mock_file_name_2', 'https://2.mock.url'),