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/control/forms/event.py

@@ -1199,6 +1199,20 @@ class MailSettingsForm(FormPlaceholderMixin, SettingsForm):
         required=False,
         widget=I18nMarkdownTextarea,
     )
+    mail_subject_order_invoice = I18nFormField(
+        label=_("Subject"),
+        required=False,
+        widget=I18nTextInput,
+        help_text=_("This will only be used if the invoice is sent to a different email address or at a different time "
+                    "than the order confirmation."),
+    )
+    mail_text_order_invoice = I18nFormField(
+        label=_("Text"),
+        required=False,
+        widget=I18nMarkdownTextarea,
+        help_text=_("This will only be used if the invoice is sent to a different email address or at a different time "
+                    "than the order confirmation."),
+    )
     mail_subject_download_reminder = I18nFormField(
         label=_("Subject sent to order contact address"),
         required=False,
@@ -1350,6 +1364,8 @@ class MailSettingsForm(FormPlaceholderMixin, SettingsForm):
         'mail_text_order_payment_failed': ['event', 'order'],
         'mail_subject_order_payment_failed': ['event', 'order'],
         'mail_text_order_custom_mail': ['event', 'order'],
+        'mail_text_order_invoice': ['event', 'order', 'invoice'],
+        'mail_subject_order_invoice': ['event', 'order', 'invoice'],
         'mail_text_download_reminder': ['event', 'order'],
         'mail_subject_download_reminder': ['event', 'order'],
         'mail_text_download_reminder_attendee': ['event', 'order', 'position'],