Convert CSS to LESS

* Rename all .css files to .less
* Start using some LESS language features to tidy up
  ext.guidedTour.less and mediawiki.libs.guiders.less
* Move common box-shadow and border-radius properties into the shared
  ext.guidedTour.mixins.less stylesheet

Change-Id: Ibe1514eef7ac1bbdddcad48cca9255960b383517
Merge "Convert CSS to LESS"