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/services/mail.py

@@ -186,10 +186,6 @@ def mail(email: Union[str, Sequence[str]], subject: str, template: Union[str, La
     headers.setdefault('X-Mailer', 'pretix')
 
     with language(locale):
-        if isinstance(context, dict) and event:
-            for k, v in event.meta_data.items():
-                context['meta_' + k] = v
-
         if isinstance(context, dict) and order:
             try:
                 context.update({