Fix focus states for 'applies to' links

- when the link has been visited and then focussed, the style was for white text on a yellow (focussed) background
- adds an explicit focus state colour to fix this

app/assets/stylesheets/components/_important-metadata.scss

@@ -31,6 +31,9 @@
 
   .app-link {
     color: govuk-colour("white");
-  }
 
+    &:focus {
+      color: $govuk-text-colour;
+    }
+  }
 }