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/webhooks.py

@@ -20,6 +20,7 @@
 # <https://www.gnu.org/licenses/>.
 #
 import django_filters
+from django.db import transaction
 from django_filters.rest_framework import DjangoFilterBackend, FilterSet
 from rest_framework import viewsets
 
@@ -48,6 +49,7 @@ class WebHookViewSet(viewsets.ModelViewSet):
         ctx['organizer'] = self.request.organizer
         return ctx
 
+    @transaction.atomic()
     def perform_create(self, serializer):
         inst = serializer.save(organizer=self.request.organizer)
         self.request.organizer.log_action(
@@ -57,6 +59,7 @@ class WebHookViewSet(viewsets.ModelViewSet):
             data=merge_dicts(self.request.data, {'id': inst.pk})
         )
 
+    @transaction.atomic()
     def perform_update(self, serializer):
         inst = serializer.save(organizer=self.request.organizer)
         self.request.organizer.log_action(
@@ -67,6 +70,7 @@ class WebHookViewSet(viewsets.ModelViewSet):
         )
         return inst
 
+    @transaction.atomic()
     def perform_destroy(self, instance):
         self.request.organizer.log_action(
             'pretix.webhook.changed',