Display cancelled letters show as failed

In the long term, we don't want to show cancelled letters. But for now,
this changes cancelled letters to display in the same way that letters
with a status of permanent-failure, since we are currently giving
letters that we want to cancel the status of permanent failure.

tests/app/main/views/test_notifications.py

@@ -195,6 +195,10 @@ def test_notification_page_shows_page_for_letter_notification(
         'permanent-failure',
         'Cancelled 1 January at 1:02am',
     ),
+    (
+        'cancelled',
+        'Cancelled 1 January at 1:02am',
+    ),
     (
         'validation-failed',
         'Cancelled 1 January at 1:02am (letter has content outside the printable area)',