Link back to uploaded letters page

If you’ve come to look at a notification via the uploaded letters page
then the ‘< back’ link should take you back there, not to the usual
activity page.

tests/app/main/views/test_notifications.py

@@ -117,6 +117,10 @@ def test_notification_status_page_respects_redaction(
         {'from_job': 'job_id'},
         partial(url_for, 'main.view_job', job_id='job_id'),
     ),
+    (
+        {'from_uploaded_letters': '2020-02-02'},
+        partial(url_for, 'main.uploaded_letters', letter_print_day='2020-02-02'),
+    ),
     (
         {'help': '0'},
         None,