Reworked css build setup to make source maps usable

Instead of a single css file containing everything, split application
styles from library styles. For library styles, concat and minify
manually rather than using vendor-supplied files since foundation
doesn't minify and also references the underlying .scss files in the
sourcemap. Skip sourcemaps altogether for vendor files.

For application styles, generate a sourcemap and make the underlying
less files available to the browser. Restify will server .less as
application/octet-stream which the browser cannot render, so
additional middlware is needed to do the right thing.