xo: enable strict mode

Signed-off-by: XhmikosR <xhmikosr@gmail.com>

scripts/js/gravity.js

@@ -5,6 +5,8 @@
  *  This file is copyright under the latest version of the EUPL.
  *  Please see LICENSE file for your rights under this license. */
 
+"use strict";
+
 function eventsource() {
   const alInfo = $("#alInfo");
   const alSuccess = $("#alSuccess");