Publish lightboxing on the message bus

With this change, images appear to lightbox instantly on my development
machine.

As per https://meta.discourse.org/t/easy-way-to-display-images-in-full-resolution/19324/13, https://meta.discourse.org/t/images-no-longer-getting-lightboxed/14965/17?u=riking, https://meta.discourse.org/t/images-no-longer-getting-lightboxed/14965/24?u=riking, https://meta.discourse.org/t/image-popover-hover-doesnt-show-except-on-pngs/18688....