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/api/serializers/item.py

@@ -441,7 +441,22 @@ class ItemCategorySerializer(I18nAwareModelSerializer):
 
     class Meta:
         model = ItemCategory
-        fields = ('id', 'name', 'internal_name', 'description', 'position', 'is_addon')
+        fields = (
+            'id', 'name', 'internal_name', 'description', 'position',
+            'is_addon', 'cross_selling_mode',
+            'cross_selling_condition', 'cross_selling_match_products'
+        )
+
+    def validate(self, data):
+        data = super().validate(data)
+
+        full_data = self.to_internal_value(self.to_representation(self.instance)) if self.instance else {}
+        full_data.update(data)
+
+        if full_data.get('is_addon') and full_data.get('cross_selling_mode'):
+            raise ValidationError('is_addon and cross_selling_mode are mutually exclusive')
+
+        return data
 
 
 class QuestionOptionSerializer(I18nAwareModelSerializer):