Lightbox fixes

Fixes for the following [from here](https://github.com/guardian/frontend/pull/5934#issuecomment-55585905)

2. Making browser window wider/thinner produces hallucinogenic effects (FF, Chrome on Mac).
3. The i icon is active while on end slate. Should be grayed out and have no hover.
4. Changing window's width/entering full screen is broken while on end slate.
8. Pressing 'i' on a keyboard shows/hides the caption - super useful! Maybe 'up', 'down' keys could be hooked up here too? - it would be consistent with some other web galleries while also make it work for that tiny minority who has 'Search for text when I start typing' on in Firefox.

// @paperboyo