Rename "css/non-critical.css" to "style.css"

[toolness]

As mentioned in #279 (comment), non-critical.css is a misnomer because it contains both critical and non-critical CSS; calling it only "non-critical" leads to lots of confusion, as evidenced by the filing of #279. I think we should just rename it to style.css or something else that indicates it is the home of all the style information on the site.

[LauraHilliger]

LOL 2 year old issue but why not: #296