Full-screen image viewing — shared across the whole site

Clicking an image in a material opens it full screen. This used to work only inside the slideshow, while elsewhere the click was intercepted by browser extensions.

header, footer, nav, aside, form, button — service areas,
images there are left alone

The rule for picking images is written the other way round: it lists the areas to leave alone, not the places where it should work. Otherwise every new template would have to be added to the list by hand — and would be forgotten.