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/main.dart

@@ -1,9 +1,9 @@
 import './reorganize_html.dart' show reorganizeHtml;
 import './theme.dart' show enableThemeChanger, setTheme;
+import './parse_sitemap.dart' show getSitemap;
 
 Future<void> main() async {
   await setTheme();
-  await reorganizeHtml().then((_) {
-    enableThemeChanger();
-  });
+  await reorganizeHtml().then((_) => enableThemeChanger());
+  await getSitemap();
 }