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/discount.py

@@ -32,6 +32,7 @@
 # distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
 # License for the specific language governing permissions and limitations under the License.
 
+from django.db import transaction
 from django_filters.rest_framework import DjangoFilterBackend, FilterSet
 from django_scopes import scopes_disabled
 from rest_framework import viewsets
@@ -64,6 +65,7 @@ class DiscountViewSet(ConditionalListView, viewsets.ModelViewSet):
             'limit_sales_channels',
         )
 
+    @transaction.atomic()
     def perform_create(self, serializer):
         serializer.save(event=self.request.event)
         serializer.instance.log_action(
@@ -78,6 +80,7 @@ class DiscountViewSet(ConditionalListView, viewsets.ModelViewSet):
         ctx['event'] = self.request.event
         return ctx
 
+    @transaction.atomic()
     def perform_update(self, serializer):
         serializer.save(event=self.request.event)
         serializer.instance.log_action(
@@ -87,6 +90,7 @@ class DiscountViewSet(ConditionalListView, viewsets.ModelViewSet):
             data=self.request.data
         )
 
+    @transaction.atomic()
     def perform_destroy(self, instance):
         if not instance.allow_delete():
             raise PermissionDenied('You cannot delete this discount because it already has '