Use Javascript strict mode

src/webui/www/private/preferences_content.html

@@ -814,6 +814,8 @@
 <div style="text-align: center; margin-top: 1em;"><input type="button" value="QBT_TR(Save)QBT_TR[CONTEXT=HttpServer]" onclick="applyPreferences();" /></div>
 
 <script>
+    'use strict';
+
     // Downloads tab
     var WatchedFoldersTable = new HtmlTable($("watched_folders_tab"));