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/locale/de/LC_MESSAGES/django.po

@@ -29915,8 +29915,8 @@ msgstr "Rechnung {invoice_number}"
 msgid ""
 "Hello,\n"
 "\n"
-"you receive this message because an order for {event} was placed by "
-"{order_email} and we have been asked to forward the invoice to you.\n"
+"please find attached a new invoice for order {code} for {event}. "
+"This order has been placed by {order_email}.\n"
 "\n"
 "Best regards,  \n"
 "\n"
@@ -29924,9 +29924,8 @@ msgid ""
 msgstr ""
 "Hallo,\n"
 "\n"
-"Sie erhalten diese Nachricht weil eine Bestellung für {event} von "
-"{order_email} aufgegeben wurde und wir darum gebeten wurden, Ihnen die "
-"Rechnung weiterzuleiten.\n"
+"anbei erhalten Sie eine neue Rechnung für Bestellung {code} für {event}. "
+"Die Bestellung wurde von {order_email} aufgegeben.\n"
 "\n"
 "Viele Grüße,  \n"
 "Das {event} Team"