Invalidate queryset caches of Task and Job model on User delete

Task and Job models have many to one relationship with User model on assignee and reviewer fields. And on deleting the parent model, these fields are set to NULL.
However due to an open issue in django-cacheops package, the related querysets are not invalidated upon deletion of parent model. Refer - Suor/django-cacheops#348.
So we have to manually invalidate the cached querysets.

cvat/apps/engine/views.py

@@ -16,6 +16,7 @@
 import cv2
 from django.db.models.query import Prefetch
 import django_rq
+from cacheops import invalidate_model
 from django.apps import apps
 from django.conf import settings
 from django.contrib.auth.models import User
@@ -1092,6 +1093,12 @@ def self(self, request):
         serializer = serializer_class(request.user, context={ "request": request })
         return Response(serializer.data)
 
+    def perform_destroy(self, instance):
+        super(UserViewSet, self).perform_destroy(instance)
+        invalidate_model(Task)
+        invalidate_model(Job)
+
+
 class RedefineDescriptionField(FieldInspector):
     # pylint: disable=no-self-use
     def process_result(self, result, method_name, obj, **kwargs):