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/control/forms/item.py

@@ -406,7 +406,6 @@ class ItemCreateForm(I18nModelForm):
 
         self.fields['tax_rule'].queryset = self.instance.event.tax_rules.all()
         change_decimal_field(self.fields['default_price'], self.instance.event.currency)
-        self.fields['tax_rule'].empty_label = _('No taxation')
         self.fields['copy_from'] = forms.ModelChoiceField(
             label=_("Copy product information"),
             queryset=self.event.items.all(),
@@ -416,6 +415,8 @@ class ItemCreateForm(I18nModelForm):
         )
         if self.event.tax_rules.exists():
             self.fields['tax_rule'].required = True
+        else:
+            self.fields['tax_rule'].empty_label = _('No taxation')
 
         if not self.event.has_subevents:
             choices = [