Make sure delete operations work as expected for Polymorphic models

While testing the PR, I discovered this bug because I couldn't delete
applications I've created. After doing some research, I figured out this
is due to a known bug on django-polymorphic. More on this issue can be
found in this issue:

jazzband/django-polymorphic#229

sponsors/models/managers.py

@@ -100,6 +100,12 @@ def list_advertisables(self):
 
 class BenefitFeatureQuerySet(PolymorphicQuerySet):
 
+    def delete(self):
+        if not self.polymorphic_disabled:
+            return self.non_polymorphic().delete()
+        else:
+            return super().delete()
+
     def from_sponsorship(self, sponsorship):
         return self.filter(sponsor_benefit__sponsorship=sponsorship).select_related("sponsor_benefit__sponsorship")
 

sponsors/models/sponsors.py

@@ -252,5 +252,9 @@ def name_for_display(self):
             name = feature.display_modifier(name)
         return name
 
+    def delete(self):
+        self.features.all().delete()
+        super().delete()
+
     class Meta(OrderedModel.Meta):
         pass