VE-1686: Loading animation

Loading animation for Venus guided by the video demo from our designers.

Oasis is largely unchanged although there is an improvement that the black overlay from the loading animation never appears at the same time as the focus widget and the toolbar never appears above a black overlay. Oasis improvements should be considered a positive side-effect and not the goal of this pull request.
And wherever that 500 would be defined please comment that it has to be the same value as defined in extensions/wikia/Venus/styles/visualeditor.scss (.5s)