portal agent: add support for submenus (#35478)

static/portal-agent/js/publik.js

@@ -51,6 +51,9 @@ $(function() {
         } else if (element.slug !== undefined) {
           $(li).find('a').addClass('icon-' + element.slug);
         }
+        if (element.sub === true) {
+          $(li).addClass('sub');
+        }
         if (window.location.href.indexOf(element.url) == 0) {
           $(li).addClass('active');
         }