Allow to import orders (#1516)

* Allow to import orders

* seats, subevents

* Plugin support

* Add docs

* Warn about lack of quota handling

* Control interface test

* Test skeleton

* First tests for the impotr columns

* Add tests for all columns

* Fix question validation

src/pretix/base/models/orders.py

@@ -2033,7 +2033,7 @@ class InvoiceAddress(models.Model):
     internal_reference = models.TextField(
         verbose_name=_('Internal reference'),
         help_text=_('This reference will be printed on your invoice for your convenience.'),
-        blank=True
+        blank=True,
     )
     beneficiary = models.TextField(
         verbose_name=_('Beneficiary'),