Replace Item.hidden_if_available with relationship to other Item (#3686)

* draft

* Implementation that is closer to old one

* Fix tests

* Add tests

* Update src/pretix/control/forms/item.py

Co-authored-by: Richard Schreiber <schreiber@rami.io>

* Review notes

---------

Co-authored-by: Richard Schreiber <schreiber@rami.io>

src/tests/presale/test_checkout.py

@@ -3336,6 +3336,33 @@ class CheckoutTestCase(BaseCheckoutTestCase, TestCase):
         assert 'Workshop 1' in response.content.decode()
         assert 'Workshop 2' in response.content.decode()
 
+    def test_set_addons_hidden_if_item_available(self):
+        with scopes_disabled():
+            self.workshopquota2 = Quota.objects.create(event=self.event, name='Workshop 1', size=5)
+            self.workshopquota2.items.add(self.workshop2)
+            self.workshopquota2.variations.add(self.workshop2a)
+            self.workshop2.hidden_if_item_available = self.workshop1
+            self.workshop2.save()
+
+            ItemAddOn.objects.create(base_item=self.ticket, addon_category=self.workshopcat, min_count=1)
+            CartPosition.objects.create(
+                event=self.event, cart_id=self.session_key, item=self.ticket,
+                price=23, expires=now() - timedelta(minutes=10)
+            )
+
+        response = self.client.get('/%s/%s/checkout/questions/' % (self.orga.slug, self.event.slug), follow=True)
+        self.assertRedirects(response, '/%s/%s/checkout/addons/' % (self.orga.slug, self.event.slug),
+                             target_status_code=200)
+        assert 'Workshop 1' in response.content.decode()
+        assert 'Workshop 2' not in response.content.decode()
+
+        self.workshopquota.size = 0
+        self.workshopquota.save()
+
+        response = self.client.get('/%s/%s/checkout/addons/' % (self.orga.slug, self.event.slug), follow=True)
+        assert 'Workshop 1' in response.content.decode()
+        assert 'Workshop 2' in response.content.decode()
+
     def test_set_addons_subevent(self):
         with scopes_disabled():
             self.event.has_subevents = True