Add link to cancel letters

Added a link to cancel letters from the letter notification pages if the
letter is still able to be cancelled. Clicking on this link will show a
confirmation box, and will then cancel the letter if the user confirms.

app/assets/stylesheets/components/stick-at-top-when-scrolling.scss

@@ -51,6 +51,10 @@
     min-height: 50px;
   }
 
+  .page-footer-delete-link-without-button {
+    margin-top: 10px;
+  }
+
   .notification-status {
     margin: 0;
   }