More forceful nw-link--no-visited-state style

I was surprised that .lbh-link--no-visited-state didn’t use an
!important flag, so would often be overridden by more specific
component styles – which is exactly the opposite of what you’d want
from a specific utility class like this.

Replacing the @extend with our own :visited definition allows us to
add the !important flag.

cobrand_hackney/static/_links.scss

@@ -22,5 +22,7 @@
 }
 
 .nw-link--no-visited-state {
-    @extend .lbh-link--no-visited-state;
+    &:visited {
+        color: lbh-colour("lbh-link") !important;
+    }
 }