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>

doc/development/api/index.rst

@@ -13,6 +13,7 @@ Contents:
    email
    placeholder
    invoice
+   invoicetransmission
    shredder
    import
    customview

doc/development/api/invoicetransmission.rst

@@ -0,0 +1,65 @@
+.. highlight:: python
+   :linenothreshold: 5
+
+Writing an invoice transmission plugin
+======================================
+
+An invoice transmission provider transports an invoice from the sender to the recipient.
+There are pre-defined types of invoice transmission in pretix, currently ``"email"``, ``"peppol"``, and ``"it_sdi"``.
+You can find more information about them at :ref:`rest-transmission-types`.
+
+New transmission types can not be added by plugins but need to be added to pretix itself.
+However, plugins can provide implementations for the actual transmission.
+Please read :ref:`Creating a plugin <pluginsetup>` first, if you haven't already.
+
+Output registration
+-------------------
+
+New invoice transmission providers can be registered through the :ref:`registry <registries>` mechanism
+
+.. code-block:: python
+
+   from pretix.base.invoicing.transmission import transmission_providers, TransmissionProvider
+
+   @transmission_providers.new()
+   class SdiTransmissionProvider(TransmissionProvider):
+       identifier = "fatturapa_providerabc"
+       type = "it_sdi"
+       verbose_name = _("FatturaPA through provider ABC")
+       ...
+
+
+The provider class
+------------------
+
+.. class:: pretix.base.invoicing.transmission.TransmissionProvider
+
+   .. autoattribute:: identifier
+
+      This is an abstract attribute, you **must** override this!
+
+   .. autoattribute:: type
+
+      This is an abstract attribute, you **must** override this!
+
+   .. autoattribute:: verbose_name
+
+      This is an abstract attribute, you **must** override this!
+
+   .. autoattribute:: priority
+
+   .. autoattribute:: testmode_supported
+
+   .. automethod:: is_ready
+
+      This is an abstract method, you **must** override this!
+
+   .. automethod:: is_available
+
+      This is an abstract method, you **must** override this!
+
+   .. automethod:: transmit
+
+      This is an abstract method, you **must** override this!
+
+   .. automethod:: settings_url