Cleanup and rework layouts/partials/css.html

[chalin]

• Use style-file fingerprinting in production only (not based on whether the site is being served or not) - Fingerprint style file in production only #92
• What's that "blah.sass" file that is being referred to?
• Consider adding PostCss processing

[kapunahelewong]

I've wondered about the blah.sass file too. And there doesn't appear to be such a file in the project at all.

[nate-double-u]

I remember discussing blah.sass when I was working on the Dex site, but I can't find reference to it in Slack (it can be so hard to search in Slack). @celestehorgan do you recall what blah.sass was about?