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/base/models/items.py

@@ -1606,6 +1606,9 @@ class Question(LoggedModel):
     :param dependency_values: The values that `dependency_question` needs to be set to for this question to be applicable.
     :type dependency_values: list[str]
     """
+    class ContainerType(models.TextChoices):
+        ORDER = "O", _("Order")
+        ORDERPOSITION = "P", _("Order position")
     TYPE_NUMBER = "N"
     TYPE_STRING = "S"
     TYPE_TEXT = "T"
@@ -1641,6 +1644,12 @@ class Question(LoggedModel):
         related_name="questions",
         on_delete=models.CASCADE
     )
+    container_type = models.CharField(
+        max_length=5,
+        choices=ContainerType.choices,
+        verbose_name=_("Asked on"),
+        default=ContainerType.ORDERPOSITION,
+    )
     question = I18nTextField(
         verbose_name=_("Question")
     )