Serve compiled CSS under /_css/

To us, these are utility files that should not compete in the path
namespace with actual pages, so we push them aside with an underscore.
For example, we can now make a `/css/` page about CSS without
conflicting with the paths to the site's CSS files.

We will do the same for our JavaScript.
Update JS and move to /_js

We updated the JavaScript libs from our CompiundJS template and moved
them to `/public/_js/`.
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