[Dart] Cleaner code, force the web page to reload on modifier code

The code in `navbar.dart' is now more declarative and cleaner.

If the `lastUpdate' variable’s content change, then it will force the
user to reload the page, bypassing their cache. This can be useful in
case of an update with which the cache could break the website.

web/dart/reorganize_html.dart

@@ -1,4 +1,5 @@
-import 'dart:html' show DivElement, Element, querySelector, querySelectorAll;
+import 'dart:html'
+    show DivElement, Element, querySelector, querySelectorAll, window;
 
 import './navbar.dart' show makeNavbar;
 
@@ -40,6 +41,13 @@ Future<void> linkifyImg() async {
 }
 
 Future<void> reorganizeHtml() async {
+  final ls = window.localStorage;
+  final lastUpdate = '20200828';
+  if (ls['last-update'] != lastUpdate) {
+    ls['last-update'] = lastUpdate;
+    window.location.reload();
+  }
+
   // Make navbar
   final navbar = await makeNavbar();