Comment-out UnCSS, move to optional

For #215 

This change comments out UnCSS support as it currently breaks our menu post-build (the UnCSS parser doesn't fully understand some of the selectors we use `.app-bar.open, .app-bar.open ~ main`). I'm suggesting we leave it in there as developers who are trying to get a high PageSpeed score are going to see the 'remove unused CSS' recommendation without this in place and it would be nice to have a fast answer they can enable.

Alternatives:

* Figure out the selectors we need to add to the UnCSS ignore list and keep it enabled by default
* Drop UnCSS completely