Expose an endpoint for retrieving image data

[Utagai]

This will benefit from the natural parallelism of how browsers make requests for URLs in the HTML of a page. Currently, we serially insert the base64 encoding of every image into the HTML file, so the page load takes a while for a large selection of images.

This gives disk access to API callers, so we must be careful to limit the surface area of the filesystem that can be touched with this endpoint.

[Utagai]

It's worth noting that we also pay for the memory that is required to hold the bytes of every image in memory. This can be quite large if we watch a significant number of images, or very large ones.