Pluggable invoice transmission methods (#5020)

* Flexible invoice transmission

* UI work

* Add peppol and output

* API support

* Profile integration

* Simplify form for individuals

* Remove sent_to_customer usage

* more steps

* Revert "Bank transfer: Allow to send the invoice direclty to the accounting department (#2975)"

This reverts commit cea6c340be.

* minor fixes

* Fixes after rebase

* update stati

* Backend view

* Transmit and show status

* status, retransmission

* API retransmission

* More fields

* API docs

* Plugin docs

* Update migration

* Add missing license headers

* Remove dead code, fix current tests

* Run isort

* Update regex

* Rebase migration

* Fix migration

* Add tests, fix bugs

* Rebase migration

* Apply suggestion from @luelista

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

* Apply suggestion from @luelista

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

* Apply suggestion from @luelista

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

* Apply suggestion from @luelista

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

* Apply suggestion from @luelista

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

* Make migration reversible

* Add TransmissionType.enforce_transmission

* Fix registries API usage after rebase

* Remove code I forgot to delete

* Update transmission status display depending on type

* Add testmode_supported

* Update src/pretix/static/pretixbase/js/addressform.js

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

* Update src/pretix/static/pretixbase/js/addressform.js

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

* Update src/pretix/static/pretixbase/js/addressform.js

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

* New mechanism for non-required invoice forms

* Update src/pretix/base/invoicing/transmission.py

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

* Declare testmode_supported for email

* Make transmission_email_other an implementation detail

* Fix failing tests and add new ones

* Update src/pretix/base/services/invoices.py

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

* Add emails to email history

* Fix comma error

* More generic default email text

* Cleanup

* Remove "email invoices" button and refine logic

* Rebase migration

* Fix edge case

---------

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

src/pretix/presale/checkoutflow.py

@@ -1130,7 +1130,7 @@ class QuestionsStep(QuestionsViewMixin, CartMixin, TemplateFlowStep):
             'invoice' in self.request.GET or
             # Checking for self.invoice_address.pk is not enough as when an invoice_address has been added and later edited to be empty, it’s not None.
             # So check initial values as invoice_form can receive pre-filled values from invoice_address, widget-data or overwrites from plug-ins.
-            is_form_filled(self.invoice_form, ignore_keys=('is_business', 'country'))
+            is_form_filled(self.invoice_form, ignore_keys=('is_business', 'country', 'transmission_type'))
         )
 
         if self.cart_customer:
@@ -1144,6 +1144,7 @@ class QuestionsStep(QuestionsViewMixin, CartMixin, TemplateFlowStep):
                         "_state_for_address": a.state_for_address,
                         "_name": a.name,
                         "is_business": "business" if a.is_business else "individual",
+                        **a.transmission_info,
                     }
                     if a.name_parts:
                         name_parts = a.name_parts
@@ -1165,7 +1166,8 @@ class QuestionsStep(QuestionsViewMixin, CartMixin, TemplateFlowStep):
 
                     for k in (
                         "company", "street", "zipcode", "city", "country", "state",
-                        "state_for_address", "vat_id", "custom_field", "internal_reference", "beneficiary"
+                        "state_for_address", "vat_id", "custom_field", "internal_reference", "beneficiary",
+                        "transmission_type",
                     ):
                         v = getattr(a, k) or ""
                         # always add all values of an address even when empty,