Sitemap query now fully asynchronous

Sitemap generation is now independent from the navbar generation, and
it will be added to the navbar only once the navbar has been created.

Also, better style for code.

web/dart/navbar.dart

@@ -28,18 +28,6 @@ Future<Element> makePages() async {
   var pages = Element.ul()
     ..attributes['id'] = 'drop-page'
     ..classes.add('dropdown');
-  await parseSitemap().then((final sitemap) => {
-        sitemap.forEach((url, name) {
-          final link = Element.li()
-            ..classes.add('dropdown-item')
-            ..insertAdjacentElement(
-                'afterBegin',
-                Element.a()
-                  ..attributes['href'] = url
-                  ..innerText = name);
-          pages.insertAdjacentElement('beforeEnd', link);
-        })
-      });
   return Element.li()
     ..append(Element.a()
       ..attributes['href'] = 'javascript:void(0)'