various gallery/lightbox fixes/tweaks

A bunch of fixes and tweaks to gallery and lightbox:
- fixing nojs gallery most-viewed link
- darkening gallery most viewed onward container border color
- darkening media margin colour
- don't toggle info panel when clicking on the info panel itself in desktop
- reload endslate responsive images when it becomes visible
- keep info button highlighted while the panel is active
- switch up/down buttons (show/hide info)
- change controls to `<button>` elements #5952 
- change control focus style (now `lighten`ed when they have focus)

![image](http://media.giphy.com/media/KNEuNnTlppwcM/giphy.gif)
Can we use [pasteup-buttons](https://github.com/guardian/pasteup-buttons/blob/master/_buttons.scss) for this?