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_invoices.py

@@ -239,6 +239,7 @@ TEST_INVOICE_RES = {
             "gross_value": "23.00",
             "tax_value": "0.00",
             "tax_name": "",
+            "tax_code": None,
             "tax_rate": "0.00"
         },
         {
@@ -255,6 +256,7 @@ TEST_INVOICE_RES = {
             'variation': None,
             "gross_value": "0.25",
             "tax_value": "0.05",
+            "tax_code": None,
             "tax_name": "",
             "tax_rate": "19.00"
         }