Enable the lightbox view for images by default

Rationale: it's a nice feature and this makes it discoverable lore easily and improves XWiki's usability overall. It can be turned off if needed.

I think the reason it wasn't on by default was because of false positives. However I remember that we implemented a way for images to be skipped by the lightbox. What I don't know is whether this has been used in all places where the images shouldn't be displayed in the lightbox.