Enabling next and previous buttons in screenshot viewer

Please consider testing and merging this.

I had to update Lightbox because the existing one did not handle repeated images well. (E.g. at https://malwr.com/analysis/NjJkZTI2YWYwZmE1NDUyMzkzZmVjZGI2MzcxNjRhNDg/ the first screenshot is the same as the 3rd one. Clicking the first one presented "Image 3 of 3".)