Fix the focus colour on edit template links

Since the focus background colour is now Design System yellow, the text
should be black for sufficent contrast.

This wasn’t happening because the `:link` selector’s definition had
greater specificity.

app/assets/stylesheets/views/template.scss

@@ -20,6 +20,10 @@
     color: $light-blue-25;
   }
 
+  &:focus {
+    color: $govuk-text-colour;
+  }
+
 }
 
 .edit-template-link-letter-contact {