Don't create IIIF Image Proxy-Service if there's only one image

[jbaiter]

Currently the app will always generate a proxy Image API info.json and reference that in the manifest for the page image.
In cases where there is only a single image available for a given page, this is not necessary and we should instead just directly put the image URL into the manifest.
In these cases we could also support non-JPEG formats like PNG, since the referenced image is no longer an IIIF Image (which mandates JPEG-support).