Reapply "Improve the performance of access checking for a specific user in CourseViewList by prefetching data"

This reverts commit d8cd6135a6.

openedx/core/djangoapps/config_model_utils/utils.py

@@ -12,14 +12,24 @@ def is_in_holdback(user):
     """
     in_holdback = False
     if user and user.is_authenticated:
-        try:
-            holdback_value = ExperimentData.objects.get(
-                user=user,
-                experiment_id=EXPERIMENT_ID,
-                key=EXPERIMENT_DATA_HOLDBACK_KEY,
-            ).value
-            in_holdback = holdback_value == 'True'
-        except ExperimentData.DoesNotExist:
-            pass
+        if 'experimentdata' in getattr(user, '_prefetched_objects_cache', {}):
+            for experiment_data in user.experimentdata_set.all():
+                if (
+                    experiment_data.experiment_id == EXPERIMENT_ID and
+                    experiment_data.key == EXPERIMENT_DATA_HOLDBACK_KEY
+                ):
+                    in_holdback = experiment_data.value == 'True'
+                    break
+        else:
+            try:
+                holdback_value = ExperimentData.objects.get(
+                    user=user,
+                    experiment_id=EXPERIMENT_ID,
+                    key=EXPERIMENT_DATA_HOLDBACK_KEY,
+                ).value
+            except ExperimentData.DoesNotExist:
+                pass
+            else:
+                in_holdback = holdback_value == 'True'
 
     return in_holdback