Add codification of tax rates (#4372)

* draft

* .

* Rebase migration

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

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

* Test, isort, flake, migration rebase

* carry data & API

* Fix failing tests

* docs fixes

* Improve validation

* Tests

* More fixes

---------

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

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',
+    'code': 'S/standard',
     'price_includes_tax': True,
     'eu_reverse_charge': False,
     'home_country': '',