improve megamenu using free/open source one

[cooked]

currently we use js "taken" from https://www.megamenu.com/articles/developers/

however it might make sense to investigate alternatives:

• https://www.jqueryscript.net/tags.php?/mega%20menu/
• https://www.jqueryscript.net/demo/Creating-An-Accessible-Mega-Menu-with-jQuery-CSS3/