rest: Always scale thumbnail's largest dimension
A good example of the difference in behaviour of the PageImages
extension is here <T156800#3087507>. The extension defers to
File#transform, which scales the largest dimension of an image when
scaling, e.g. if an image is 1000px x 2000px and the request is for a
thumbnail "of 1800px", then the thumbnail will be 900px x 1800px.