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/tests/api/conftest.py

@@ -192,7 +192,7 @@ def subevent2(event2, meta_prop):
 @pytest.fixture
 @scopes_disabled()
 def taxrule(event):
-    return event.tax_rules.create(name="VAT", rate=19, code="S/standard")
+    return event.tax_rules.create(name="VAT", rate=19, code="S/standard", default=True)
 
 
 @pytest.fixture

src/tests/api/test_orders.py

@@ -50,7 +50,7 @@ def item2(event2):
 
 @pytest.fixture
 def taxrule(event):
-    return event.tax_rules.create(rate=Decimal('19.00'), code="S/standard")
+    return event.tax_rules.create(rate=Decimal('19.00'), code="S/standard", default=True)
 
 
 @pytest.fixture
@@ -1351,7 +1351,7 @@ def test_order_mark_canceled_pending(token_client, organizer, event, order):
 @pytest.mark.django_db
 def test_order_mark_canceled_pending_fee_with_tax(token_client, organizer, event, order, taxrule):
     djmail.outbox = []
-    event.settings.tax_rate_default = taxrule
+    event.settings.tax_rule_cancellation = "default"
     resp = token_client.post(
         '/api/v1/organizers/{}/events/{}/orders/{}/mark_canceled/'.format(
             organizer.slug, event.slug, order.code

src/tests/api/test_taxrules.py

@@ -31,6 +31,7 @@ TEST_TAXRULE_RES = {
     'keep_gross_if_rate_changes': False,
     'name': {'en': 'VAT'},
     'rate': '19.00',
+    'default': True,
     'code': 'S/standard',
     'price_includes_tax': True,
     'eu_reverse_charge': False,
@@ -80,6 +81,45 @@ def test_rule_create(token_client, organizer, event):
     assert str(rule.home_country) == "DE"
 
 
+@pytest.mark.django_db
+def test_rule_create_auto_default(token_client, organizer, event):
+    resp = token_client.post(
+        '/api/v1/organizers/{}/events/{}/taxrules/'.format(organizer.slug, event.slug),
+        {
+            "name": {"en": "VAT", "de": "MwSt"},
+            "rate": "19.00",
+            "price_includes_tax": True,
+            "eu_reverse_charge": False,
+            "home_country": "DE",
+        },
+        format='json'
+    )
+    assert resp.status_code == 201
+    rule = TaxRule.objects.get(pk=resp.data['id'])
+    assert rule.default
+
+
+@pytest.mark.django_db
+def test_rule_create_only_one_default(token_client, taxrule, organizer, event):
+    assert taxrule.default
+    resp = token_client.post(
+        '/api/v1/organizers/{}/events/{}/taxrules/'.format(organizer.slug, event.slug),
+        {
+            "name": {"en": "VAT", "de": "MwSt"},
+            "rate": "19.00",
+            "price_includes_tax": True,
+            "eu_reverse_charge": False,
+            "home_country": "DE",
+            "default": True,
+        },
+        format='json'
+    )
+    assert resp.status_code == 201
+
+    taxrule.refresh_from_db()
+    assert not taxrule.default
+
+
 @pytest.mark.django_db
 def test_rule_update(token_client, organizer, event, taxrule):
     resp = token_client.patch(