About page: Resposive images and CDN media URLs.

This adds `srcset` and `sizes` to the about page images, including
art direction, via `<picture>` for the lead Twenty Seventeen image.

This also replaces URLs for all images and feature videos to their
CDN locations on s.w.org.

Props helen, pento, joemcgill.
Fixes #38616.
Built from https://develop.svn.wordpress.org/trunk@39514


git-svn-id: https://core.svn.wordpress.org/trunk@39454 1a063a9b-81f0-0310-95a4-ce76da25c4cd