lightbox: Add "download" button in bottom app bar #42
Description
The bottom app bar currently has just one button, to copy a link to the image. It should also have a button to download the image, in the way that feels most familiar for the platform.
I didn't include this in the prototype because I think it'll involve a network request, like the one we do in the RN app's api.tryGetFileTemporaryUrl. As usual for network requests, that means we should consider what to do when the request succeeds, fails, or goes on forever without succeeding or failing.