Order import: Allow to create multiple multi-ticket orders (#5304)

* Order import: Allow to create multiple multi-ticket orders

* Update src/pretix/base/modelimport_orders.py

* Fix failing test

src/pretix/base/modelimport.py

@@ -111,6 +111,13 @@ class ImportColumn:
         """
         return gettext_lazy('Keep empty')
 
+    @property
+    def help_text(self):
+        """
+        Additional description of the column
+        """
+        return None
+
     def __init__(self, event):
         self.event = event