Turn screen.css into screen.less

* Separate external links into own less file to showcase how
many unnecessary css rules we have
* Introduce new mixins to allow embedding for list-style-image
* Replace use of /* embed */ with mediawiki mixins

Change-Id: If79d2d1f72579627597b291eec759441d6763c34
Introduce .transition mixin

Cleanup existing transition rules for readability

Change-Id: I2be09a7a8e7eeb42d8bf80cb033c81867da108a5