Convert CSS to LESS

* Rename all .css files to .less
* Start using some LESS language features to tidy up
  ext.gettingstarted.return.less and
  ext.gettingstarted.taskToolbar.less
* Remove the unused ext.gettingstarted.css file

Change-Id: I09e81df3a945213727a5c212bf001d728e7a3141
Merge "Convert CSS to LESS"