responsive images: srcset support

[amitu]

https://kellysutton.com/2016/06/23/boiling-the-ocean-with-markup.html

We can follow the techniques described in the blog post. We may have to do some of it using FPM as well (like dynamic/on demand or build time image resizing).

Also this: https://jfhr.me/optimizing-images-with-the-html-picture-tag/

And this: https://github.com/Josh-McFarlin/remix-image