Page images should use modern loading methods

[eyeseast]

Currently, each page in a document loads up to five versions of the page image -- thumbnail, small, normal, large, extra large -- until it finds the biggest one that fits. This uses more bandwidth than necessary and results in a bad experience in low-bandwidth settings.

There are now better ways to load images that match the browser size, and we should use them.

https://developer.mozilla.org/en-US/docs/Learn/HTML/Multimedia_and_embedding/Responsive_images

See also #197