[FEATURE] Properly cache the youtube thumbnail and deliver that as a picture tag (heavy performance improvement)

Change the way the YouTube API is called to RequestFactory,
because we do not want to care manually what method of
fetching urls is available on the server.

Feature layout settings

See merge request !5

- rewrote sgYoutubeLightbox as ES6 class
- updated UPGRADE.md for future v5 release

Feature modernize javascript

See merge request !4