Option to precache preprocessed images

[ebrahimebrahim]

In order to have a smooth experience viewing preprocessed images from subject to subject, it would be good to have a button that downloads all the images into the browser's storage, so that they can then be previewed quickly. Maybe it takes 20 minutes to download everything-- that's okay and probably much better than waiting 5 or 10 seconds for each image one at a time.

[jjnesbitt]

This will probably involve setting up those presigned URLs to include Cache-Control headers in the reponse, to take advantage of normal browser cache.