Add full screen images support

[FDelporte]

Allow all images to be displayed full screen / enlarged.
E.g. on showcase screen, books (cover image), ...

[dlemmermann]

Normally this is done via a Lightbox concept, but the problem is that our "pages" are very tall and centering a Lightbox would mean showing it in a place that is not visible in the viewport. We might need some browser-based / javascript approach. Assigning this to @FlorianKirmaier as he is the one with the required skillset to solve this issue.