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/control/forms/event.py

@@ -60,12 +60,11 @@ from i18nfield.forms import (
 from pytz import common_timezones
 
 from pretix.base.channels import get_all_sales_channels
-from pretix.base.email import get_available_placeholders
 from pretix.base.forms import I18nModelForm, PlaceholderValidator, SettingsForm
-from pretix.base.forms.widgets import format_placeholders_help_text
 from pretix.base.models import Event, Organizer, TaxRule, Team
 from pretix.base.models.event import EventFooterLink, EventMetaValue, SubEvent
 from pretix.base.reldate import RelativeDateField, RelativeDateTimeField
+from pretix.base.services.placeholders import FormPlaceholderMixin
 from pretix.base.settings import (
     COUNTRIES_WITH_STATE_IN_ADDRESS, DEFAULTS, PERSON_NAME_SCHEMES,
     PERSON_NAME_TITLE_GROUPS, validate_event_settings,
@@ -503,7 +502,7 @@ class EventSettingsValidationMixin:
                 del self.cleaned_data[field]
 
 
-class EventSettingsForm(EventSettingsValidationMixin, SettingsForm):
+class EventSettingsForm(EventSettingsValidationMixin, FormPlaceholderMixin, SettingsForm):
     timezone = forms.ChoiceField(
         choices=((a, a) for a in common_timezones),
         label=_("Event timezone"),
@@ -593,6 +592,10 @@ class EventSettingsForm(EventSettingsValidationMixin, SettingsForm):
         'og_image',
     ]
 
+    base_context = {
+        'frontpage_text': ['event'],
+    }
+
     def _resolve_virtual_keys_input(self, data, prefix=''):
         # set all dependants of virtual_keys and
         # delete all virtual_fields to prevent them from being saved
@@ -682,6 +685,9 @@ class EventSettingsForm(EventSettingsValidationMixin, SettingsForm):
             else:
                 self.initial[virtual_key] = 'do_not_ask'
 
+        for k, v in self.base_context.items():
+            self._set_field_placeholders(k, v)
+
     @cached_property
     def changed_data(self):
         data = []
@@ -932,7 +938,7 @@ def contains_web_channel_validate(val):
         raise ValidationError(_("The online shop must be selected to receive these emails."))
 
 
-class MailSettingsForm(SettingsForm):
+class MailSettingsForm(FormPlaceholderMixin, SettingsForm):
     auto_fields = [
         'mail_prefix',
         'mail_from_name',
@@ -1344,17 +1350,6 @@ class MailSettingsForm(SettingsForm):
         'mail_attach_ical_description': ['event', 'event_or_subevent'],
     }
 
-    def _set_field_placeholders(self, fn, base_parameters):
-        placeholders = get_available_placeholders(self.event, base_parameters)
-        ht = format_placeholders_help_text(placeholders, self.event)
-        if self.fields[fn].help_text:
-            self.fields[fn].help_text += ' ' + str(ht)
-        else:
-            self.fields[fn].help_text = ht
-        self.fields[fn].validators.append(
-            PlaceholderValidator(['{%s}' % p for p in placeholders.keys()])
-        )
-
     def __init__(self, *args, **kwargs):
         self.event = event = kwargs.get('obj')
         super().__init__(*args, **kwargs)