internal/static: bundle and minifiy css files during static build
internal/static: bundle and minifiy css files during static build

The last hurdle to minifying out CSS was the legacy stylesheets in
static/legacy. Now that they are removed we can bundle and minify
stylesheets for each page in a <page>.min.css file.

Change-Id: I0f26458cc85f9381f8bd8a0e225df79268594d0f
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/344399
Trust: Jamal Carvalho <e0195770807aa8c82b0b128d9c0423b5ad035172@golang.org>
Run-TryBot: Jamal Carvalho <e0195770807aa8c82b0b128d9c0423b5ad035172@golang.org>
TryBot-Result: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>

golang/pkgsite,golang/pkgsite,golang/pkgsite,golang/pkgsite