Stop flicker when hovering postage on precompiled

We were removing the border to ‘unfold’ the corner of the page. This was
causing the size of the element to shrink.

This meant that it you hovered the bottom 1px of the element it would
cycle in and out of the hover state as fast as the browser could render
it.

This commit fixes that by making the border transparent, rather than
removing it.

app/assets/stylesheets/components/letter.scss

@@ -85,7 +85,7 @@ $iso-paper-ratio: 141.42135624%;
     .letter-sent &:hover {
 
       background-color: transparent;
-      border: none;
+      border-color: transparent;
       box-shadow: none;
       background-image: file-url('envelope-fold.svg');