Fix breadcrumbs' layout in Firefox

The negative margin added to the breadcrumb links to improve the
clickable area introduced a bug in firefox, where the link text wraps to
a new line.

By adding `display: inline-block` we make sure the link doesn't wrap to
a new line like in other browsers.

app/assets/stylesheets/govuk-component/mixins/_touch-friendly-links.scss

@@ -6,5 +6,6 @@
     padding: $padding;
     margin: -1 * $padding;
     outline-color: transparent;
+    display: inline-block;
   }
 }