Add all the yelp-xsl css as sass

Think this will be the easiest place to work from for restyling the
css. The yelp-xsl css covers all the elements of the html, and
should be pretty much what the stock gnome styling looks like.

I'm sure we can dump a lot of it, but seeing how upstream fit the
css with the html content will hopefully be useful.
[endlessm/eos-shell#4154]