Allow to add declaration of accessibility (#5140)

* Allow to add declaration of accessibility

* add fallback for empty accessibility_title

* unify label format (not "Title for")

* move title to top and set helptext before text

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Co-authored-by: Richard Schreiber <schreiber@rami.io>

src/pretix/api/serializers/organizer.py

@@ -426,6 +426,9 @@ class OrganizerSettingsSerializer(SettingsSerializer):
         'organizer_logo_image_inherit',
         'organizer_logo_image',
         'privacy_url',
+        'accessibility_url',
+        'accessibility_title',
+        'accessibility_text',
         'cookie_consent',
         'cookie_consent_dialog_title',
         'cookie_consent_dialog_text',