Introduce country-specific address validation (#2945)

Co-authored-by: Richard Schreiber <schreiber@rami.io>

src/pretix/presale/forms/checkout.py

@@ -113,6 +113,7 @@ class ContactForm(forms.Form):
 class InvoiceAddressForm(BaseInvoiceAddressForm):
     required_css_class = 'required'
     vat_warning = True
+    address_validation = True
 
     def __init__(self, *args, **kwargs):
         allow_save = kwargs.pop('allow_save', False)