Redo how we do lightboxes for images

[benlk]

• remove or merge the Navis Slideshows functionality deeper into Largo

[MirandaEcho]

Lightboxes redo

[benlk]

See also:

• not enqueueing unneeded stylesheets Don't enqueue unneeded stylesheets for Navis #1113
• conditionally enqueue navis assets: Conditionally enqueue Navis materials #1760

[benlk]

It might be better to just strip our functionality-related modifications to the Gallery block/shortcode/widget entirely. Let people who want to have non-WordPress behavior use plugins that are built for galleries and slideshows and so on.