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/pretix/base/migrations/0249_hidden_if_item_available.py

@@ -0,0 +1,22 @@
+# Generated by Django 4.2.4 on 2023-10-30 11:50
+
+import django.db.models.deletion
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+    dependencies = [
+        ("pretixbase", "0248_item_free_price_suggestion"),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name="item",
+            name="hidden_if_item_available",
+            field=models.ForeignKey(
+                null=True,
+                on_delete=django.db.models.deletion.SET_NULL,
+                to="pretixbase.item",
+            ),
+        ),
+    ]