add nested menus capability

assets/js/header.js

@@ -0,0 +1,11 @@
+function header_toggle_nested_menu(element) {
+  if (!element) {
+    throw new Error("Could not find button")
+  }
+  let parent_element = element.parentElement;
+  if(!parent_element) {
+    throw new Error("Could not get parent element from button")
+  }
+  let nested_menu = parent_element.querySelector(".header-nested-menu")
+  nested_menu.classList.toggle('hidden');
+}