Image Caching

[rowanc1]

Currently when we build a site or document we have to fait for image optimizations to complete. These image optimizations are completed in each export process, and are not shared. For a site build (e.g. clean) this can mean a significant slow down, especially for GIFs that are optimized to WebP.

Now that we have a cache folder, we should use that space to optimize the images and make the clean not wipe these out as they should be deterministic and based entirely on the image hash (not the filename at all). That would change build-time to be copying from the image directory into the appropriate folder, should ensure the sha is more consistent between builds, and just generally speed things up!

cc @fwkoch