Lazy-load offscreen images

[nelsonaponte]

All images not immediately displayed on page load should be deferred (lazy-loaded) on the following pages to decrease loading times and increase user experience:

1. https://www.openhab.org
2. https://www.openhab.org/blog/
3. https://www.openhab.org/download/
4. https://www.openhab.org/about/showcase.html