Build a minified version of each theme file using cssnano

[pugnascotia]

Like the summary says. I thought it might be slow to re-run PostCSS but in practice I couldn't tell. We still only run sass once.

I struggled to generate source maps, so if we want them then I'd appreciate some input.

[chandlerprall]

LGTM. Pulled and ran the compile-scss script, verified output looks correct (retains autoprefixef as it should, etc)

[chandlerprall]

Please commit the modified yarn.lock, then this looks great :)

[pugnascotia]

After a discussion with @chandlerprall, we're going to merge the changes as-is.

[chandlerprall]

My yarn has a difference in opinion, going to go with @pugnascotia's original yarn.lock changes.

all LGTM!