Improve processing of uploaded images

As mentioned in dev chat on May 15 there are quite a few “stalled” tickets on Trac dealing with image processing/resizing after uploading:

All of these, and more, can be “unblocked” by: Save progress of intermediate image creation after upload #40439.

Fixing this will also enable “refreshing” of the way we do responsive images, and will enable improvements in how we generally work with images in the block editor. Then we will be able to enhance the Image block and make it “just work” regardless of the context, see Enhancement: images handling and more importantly https://github.com/WordPress/gutenberg/issues/6177#issuecomment-442953527.

In short: fixing the above tickets will greatly improve user experience especially when uploading large images or the server is slow/busy. It will also open the way for enhancing how we work with images in the editor, and let us show better fitting images on the front-end.

#media, #upload