API: Make all mutating endpoints transactional (#6430)

All of our API endpoints that do something in the system do at least two
SQL queries, one for the actual change and one for the log entry. Often
many more. We want all of this to happen in a transaction so we know an
API call was executed or not at all, not half-way.

src/pretix/api/views/organizer.py

@@ -394,6 +394,7 @@ class TeamViewSet(viewsets.ModelViewSet):
         )
         return inst
 
+    @transaction.atomic()
     def perform_destroy(self, instance):
         instance.log_action('pretix.team.deleted', user=self.request.user, auth=self.request.auth)
         instance.delete()
@@ -693,6 +694,7 @@ class MembershipTypeViewSet(viewsets.ModelViewSet):
         ctx['organizer'] = self.request.organizer
         return ctx
 
+    @transaction.atomic()
     def perform_destroy(self, instance):
         if not instance.allow_delete():
             raise PermissionDenied("Can only be deleted if unused.")
@@ -833,6 +835,7 @@ class SalesChannelViewSet(viewsets.ModelViewSet):
         )
         return inst
 
+    @transaction.atomic()
     def perform_destroy(self, instance):
         if not instance.allow_delete():
             raise PermissionDenied("Can only be deleted if unused.")