Allow template syntax in event text (Z#23140046) (#3815)

* remove duplicate context generation

* allow text templates in frontpage_text

* refactor: move placeholder functionality to separate file

* fix wrong class name, code style

* update year in license header

* undo license header update

* use new function name

* render only the placeholders that are actually used in the message

* refactoring

* add str(...) call

* Update doc/development/api/placeholder.rst

Co-authored-by: Raphael Michel <michel@rami.io>

* rename register_mail_placeholders to register_template_placeholders
(deprecate old name)

* isort

* add signals to docs

---------

Co-authored-by: Raphael Michel <michel@rami.io>

src/pretix/base/models/orders.py

@@ -1090,9 +1090,6 @@ class Order(LockModel, LoggedModel):
         if not self.email and not (position and position.attendee_email):
             return
 
-        for k, v in self.event.meta_data.items():
-            context['meta_' + k] = v
-
         with language(self.locale, self.event.settings.region):
             recipient = self.email
             if position and position.attendee_email:
@@ -2650,9 +2647,6 @@ class OrderPosition(AbstractPosition):
         if not self.attendee_email:
             return
 
-        for k, v in self.event.meta_data.items():
-            context['meta_' + k] = v
-
         with language(self.order.locale, self.order.event.settings.region):
             recipient = self.attendee_email
             try: