Tell template preview to allow international letters

If a service has permission to send international letters then it should
tell template preview, so that template preview knows what rule to
apply when it’s validating the address of the letter.

Depends on:
- [ ] https://github.com/alphagov/notifications-template-preview/pull/445

app/celery/letters_pdf_tasks.py

@@ -40,6 +40,7 @@ from app.letters.utils import (
     get_file_names_from_error_bucket,
 )
 from app.models import (
+    INTERNATIONAL_LETTERS,
     KEY_TYPE_TEST,
     NOTIFICATION_CREATED,
     NOTIFICATION_DELIVERED,
@@ -227,6 +228,9 @@ def sanitise_letter(self, filename):
             kwargs={
                 'notification_id': str(notification.id),
                 'filename': filename,
+                'allow_international_letters': notification.service.has_permission(
+                    INTERNATIONAL_LETTERS
+                ),
             },
             queue=QueueNames.SANITISE_LETTERS,
         )