Add skip link data module to initialise JS

Author: Vanita Barrett

lib/source/layouts/core.erb

@@ -27,7 +27,7 @@
 
     <div class="app-pane">
       <div class="app-pane__header toc-open-disabled">
-        <a href="#content" class="govuk-skip-link">Skip to main content</a>
+        <a href="#content" class="govuk-skip-link" data-module="govuk-skip-link">Skip to main content</a>
 
         <%= partial 'layouts/header' %>
       </div>