Add support for selecting a responsive image style for the Photoswipe modal image, similar to the Thumbnail image style option

Provided implementation for supporting responsive image style for Photoswipe modal image . Please review.

Thanks for adding this feature @kostiantyn. I was wondering just now what the difference is between the View modes "Photoswipe" or "Photoswipe Responsive", so I searched and found your issue.

I am also not sure how to set up "Photoswipe Responsive", so perhaps you could consider add a small description in the README as well, outlining the basic steps to set up? Or if you leave a comment here, I will gladly go through it, and add it in the README via the MR.

On a side note, I also hope captions can be made to work at some point, since that's also pretty much a requirement in modern web sites -- to have attribution of the images, but that's another issue ( 🐛 photo filed - caption Needs work ).