Support linking the image for Drupal 8

The Drupal 8 port is almost done. There's just one small bit that needs to be fixed, and that's the possibility to make the image link to the content or the file. This was possible in D7, but has not been ported to D8 yet. See: https://www.drupal.org/node/2684631#comment-11046237

Problem description

The D7 version of the module uses theme_image_formatter but in D8, it requires an ImageItem object which we don't have. Instead of that, I'm using theme_image_style, so the question is what is the best way to generate a link with the themed image object.