Order-level questions (#6471)

* New CheckoutSession model, created and deleted throught cart lifetime but only used for order-level question answers so far
* Order-level QuestionAnswers (relations to CheckoutSession / Order)
* New container_type field on Question model to specify whether Question belongs to order or orderposition
* Order-level questions are currently experimental, UI is hidden behind feature flag

---------

Co-authored-by: Richard Schreiber <schreiber@pretix.eu>

src/pretix/control/views/orders.py

@@ -2226,6 +2226,10 @@ class OrderModifyInformation(OrderQuestionsViewMixin, OrderView):
     only_user_visible = False
     all_optional = True
 
+    @property
+    def order_question_container(self):
+        return self.order
+
     def get_context_data(self, **kwargs):
         ctx = super().get_context_data(**kwargs)
         ctx['other_form'] = self.other_form
@@ -2587,7 +2591,11 @@ class AnswerDownload(EventPermissionRequiredMixin, OrderViewMixin, ListView):
         answid = kwargs.get('answer')
         token = request.GET.get('token', '')
 
-        answer = get_object_or_404(QuestionAnswer, orderposition__order=self.order, id=answid)
+        answer = get_object_or_404(
+            QuestionAnswer,
+            Q(orderposition__order=self.order) | Q(order=self.order),
+            id=answid
+        )
         if not answer.file:
             raise Http404()
         if not check_token(request, answer, token):
@@ -2597,7 +2605,7 @@ class AnswerDownload(EventPermissionRequiredMixin, OrderViewMixin, ListView):
         resp = FileResponse(answer.file, content_type=ftype or 'application/binary')
         resp['Content-Disposition'] = 'attachment; filename="{}-{}-{}-{}"'.format(
             self.request.event.slug.upper(), self.order.code,
-            answer.orderposition.positionid,
+            answer.orderposition.positionid if answer.orderposition else '',
             os.path.basename(answer.file.name).split('.', 1)[1]
         )
         return resp