Orders: Fix N+1 query issues on order detail and change page (#6390)

* rders: Fix N+1 query issues on order detail and change page

There is one I couldn't fix: Loading the list of tax rules for every
select box on the OrderChange page. Unfortunately, Django has a
cache-breaking .all() in ModelChoiceField and that would need nasty
patching that didn't feel worth it

* Fix isort

* Fix N+1 query in gate call

* Fix leftovers

* Add local cache for objects referenced in log entries

* Update src/pretix/control/views/orders.py

src/pretix/control/forms/orders.py

@@ -562,9 +562,7 @@ class OrderPositionChangeForm(forms.Form):
         if instance.addon_to_id:
             del self.fields['operation_split']
 
-        if not instance.seat and not (
-                instance.item.seat_category_mappings.filter(subevent=instance.subevent).exists()
-        ):
+        if not instance.seat and not instance._seat_allowed:
             del self.fields['seat']
 
         choices = [