Add envelope for international letters

International letters aren’t sent by first or second class post. In
keeping with the little touch of skeumorphism, let’s label them with the
commonly recognised marker of international mail instead.

app/assets/stylesheets/components/letter.scss

@@ -70,6 +70,10 @@ $iso-paper-ratio: 141.42135624%;
       background-image: file-url('envelope-2nd-class.svg');
     }
 
+    &-international {
+      background-image: file-url('envelope-international.svg');
+    }
+
     .letter-sent &:hover {
       background-color: transparent;
       border-color: transparent;