Update utils with to make letters unclickable

Brings in:
- [ ] https://github.com/alphagov/notifications-utils/pull/159

Means:
- renaming a bunch of things
- removing any CSS that targeted the link, and making it target the
  containing element too

app/assets/stylesheets/components/letter.scss

@@ -8,7 +8,7 @@ $iso-paper-ratio: 141.42135624%;
 
 .letter {
 
-  padding: 0;
+  padding: $iso-paper-ratio 0 0 0;
   margin: 0 0 $gutter 0;
   position: relative;
   background: $panel-colour;
@@ -28,16 +28,6 @@ $iso-paper-ratio: 141.42135624%;
     width: 0px;
   }
 
-  a {
-    display: block;
-    overflow: hidden;
-    width: 100%;
-    height: 0;
-    padding: $iso-paper-ratio 0 0 0;
-    position: relative;
-    z-index: 20;
-  }
-
   img {
     display: block;
     width: 100%;