MAPG-89 adaptations in HtmlContent and index.php

public/index.php

@@ -47,5 +47,6 @@ if ($match !== null) {
     }
 }
 
+$content = new MapGuesser\Response\HtmlContent('error/404');
 header('Content-Type: text/html; charset=UTF-8', true, 404);
-require ROOT . '/views/error/404.php';
+echo $content->render();