feat: system alert message (#3139)

lib/html.php

@@ -17,6 +17,7 @@ function render(string $template, array $context = []): string
     if ($template === 'base.html.php') {
         throw new \Exception('Do not render base.html.php into itself');
     }
+    $context['system_message'] = Configuration::getConfig('system', 'message');
     $context['page'] = render_template($template, $context);
     return render_template('base.html.php', $context);
 }