Update our Sass build process

It will now use our internal PHP Sass Compiler (that happens to be
bundled with the Custom CSS module), so there's one less external
dependency to worry about needing to install and have configured
identically.

Also renames the file, as it’s not using CSSTidy any longer, and uses a
custom style generator class for the PHP Sass to match core CSS Styles.
Update all the generated CSS.

The PHP Sass processor does a few things different now that it’s not
passing through CSSTidy.  Leading zeros are left on, quotes are left as
their original versions, some space is left when using rgba and the
like … good things, all.

Also, the `pointer-events` property isn’t stripped out any longer.