Add sales channels (#1103)

- [x] Data model
- [x] Enforce constraint
- [x] Filter order list
- [x] Set channel on created order
- [x] Products API
- [x] Order API
- [x] Tests
- [x] Filter reports
- [x] Resellers
- [ ] deploy plugins
  - [ ] posbackend
  - [ ] resellers
  - [ ] reports
- [x] Ticketlayouts
- [x] Support in pretixPOS

src/pretix/control/views/item.py

@@ -834,7 +834,10 @@ class ItemUpdateGeneral(ItemDetailMixin, EventPermissionRequiredMixin, UpdateVie
     def plugin_forms(self):
         forms = []
         for rec, resp in item_forms.send(sender=self.request.event, item=self.item, request=self.request):
-            forms.append(resp)
+            if isinstance(resp, (list, tuple)):
+                forms.extend(resp)
+            else:
+                forms.append(resp)
         return forms
 
     def get_success_url(self) -> str: