Add progressive enhancement to service switcher

- by default, the menu is open
- if Javascript is enabled/loaded, the links are hidden, and visual cues (▶) to
  show that it can be opened are added
- clicking it opens and closes it

app/assets/javascripts/dropdown.js

@@ -0,0 +1,18 @@
+(function(Modules) {
+  "use strict";
+
+  Modules.Dropdown = function() {
+
+    this.start = function(component) {
+
+      $('.dropdown-toggle', component)
+        .on(
+          'click', () => $(component).toggleClass('js-closed')
+        )
+        .trigger('click');
+
+    };
+
+  };
+
+})(window.GOVUK.Modules);