Fix invisible focused item text

GOV.UK Template hanged the colour of text in focused links in
79466a489c

It was done with greater specificity than before. This means that the
way we were previously overriding the focus colour (for links with a
dark background) no longer works.

This commit makes our override more specific, so that it works again.

app/assets/stylesheets/components/pill.scss

@@ -80,7 +80,9 @@
       text-decoration: underline;
     }
 
-    &:hover {
+    &:hover,
+    &:focus,
+    &:link:focus {
       color: $light-blue-25;
     }