Cross selling (#4185)

Product categories can now be marked as "cross-selling categories", causing them to 
appear in the add-on checkout step as additional recommendations, depending on 
their cross-selling visibility (always, only if certain products are already in the cart, or 
only if they qualify for a discount according to discount rules).

---------

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

src/pretix/base/migrations/0271_itemcategory_cross_selling.py

@@ -0,0 +1,32 @@
+# Generated by Django 4.2.11 on 2024-05-27 13:19
+
+from django.db import migrations, models
+
+import pretix.base.models.orders
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ("pretixbase", "0270_historicpassword"),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name="itemcategory",
+            name="cross_selling_condition",
+            field=models.CharField(null=True, max_length=10),
+        ),
+        migrations.AddField(
+            model_name="itemcategory",
+            name="cross_selling_mode",
+            field=models.CharField(null=True, max_length=5),
+        ),
+        migrations.AddField(
+            model_name="itemcategory",
+            name="cross_selling_match_products",
+            field=models.ManyToManyField(
+                related_name="matched_by_cross_selling_categories", to="pretixbase.item"
+            ),
+        ),
+    ]