Add lazy loading utils & implement in Scroll.js

[devenini]

Lazy loading utils

• Better requests management
  • Stores all loaded images in an array
  • Check if the image has already been loaded before loading it
• Better naming
  • toggleLazy => lazyLoad
  • data-scroll-lazy=“assets/images/foo.jpg” => data-src=“assets/images/foo.jpg”
• Add SVG src example to set an image ratio

I also fixed wrong stylesheet ids in form.html & images.html