This commit adds support for unfurling static image URLs (not GIFs, not animated WebPs), such as https://placehold.co/600x400@2x.png. It also compresses images before returning them as data URIs to clients.
About compression: the compression strategy leverages the existing function images.CompressToFileLimits. A more comprehensive logic to consider the possibility of multiple image URLs being unfurled simultaneously is yet to be implemented.
Closes#3761
