VE-1682: VEnus

Load VisualEditor in the Venus skin.
@kvas-damian @rafalkalinski VisualEditor copies all stylesheets from the main document and loads them into dialog iframes (all VE dialogs are inside iframes). body:not(.venus-preview) was matching for the iframe body tags, so I've made 2 changes: 

1. It looks like you were using not(.venus-preview) because of a similar iframe problem. I checked out the implementation and you don't use the classname 'skin-venus' on the body tag in the iframe. Neither does VE, so I've changed the selector to body.skin-venus.

2. I've nested the selectors for DRYness.