New data model for default tax rule and new options for cancellation fees (#4962)

* New data model for default tax rule

* Remove misleading empty label when field is not optional

* Allow to split cancellation fee

* Fix API and tests

* Update migration

* Update src/tests/api/test_taxrules.py

Co-authored-by: luelista <weller@rami.io>

* Update src/tests/api/test_taxrules.py

Co-authored-by: luelista <weller@rami.io>

* Review note

* Update src/pretix/base/models/tax.py

Co-authored-by: luelista <weller@rami.io>

* Flip API behaviour for default

* Fix failing tests

* Fix failing test

* Split migration

---------

Co-authored-by: luelista <weller@rami.io>

src/pretix/base/models/tax.py

@@ -377,9 +377,20 @@ class TaxRule(LoggedModel):
                     'if configured above.'),
     )
     custom_rules = models.TextField(blank=True, null=True)
+    default = models.BooleanField(
+        verbose_name=_('Default'),
+        default=False,
+    )
 
     class Meta:
         ordering = ('event', 'rate', 'id')
+        constraints = [
+            models.UniqueConstraint(
+                fields=["event"],
+                condition=models.Q(default=True),
+                name="one_default_per_event",
+            ),
+        ]
 
     class SaleNotAllowed(Exception):
         pass
@@ -394,7 +405,7 @@ class TaxRule(LoggedModel):
             and not OrderFee.objects.filter(tax_rule=self, order__event=self.event).exists()
             and not OrderPosition.all.filter(tax_rule=self, order__event=self.event).exists()
             and not self.event.items.filter(tax_rule=self).exists()
-            and self.event.settings.tax_rate_default != self
+            and not (self.default and self.event.tax_rules.filter(~models.Q(pk=self.pk)).exists())
         )
 
     @classmethod