Extend img include for multiple resolutions

[white-gecko]

Add support for multiple resolutions of an image:

cf.

 <img class="js-load" data-src="{{ image.link }}" data-sizes="auto"
              data-srcset="{{ image.link }} 800w, {{ image.link }} 1980w"
              width="6240" height="4160" style="max-width:6240px" alt="{{ image.description }}">