Add option to use scaled image and lightbox and full size for download

[arnowelzel]

Also see https://wordpress.org/support/topic/option-to-load-custom-image-size-instead-of-full/ or https://wordpress.org/support/topic/download-image-in-full-size-not-scaled/

Some users might want to display a scaled image (e.g. "medium" or "large") in the lightbox and use the full size image only for downloads.

Caveats:

• This also means the existing URLs in the image links need to be changed and there need to be additinal data attributes to indicate the "full" size URL for the download button
• Image sizes in WordPress are only defined as absolute maximum values. The real image sizes generated for an indiviual image are not fixed. So one can not just simple replace the URL by adding "-WIDTHxHEIGHT" to it but instead all existing scaled versions have to be scanned in the upload folder and compared against the predefined sizes of the current theme.