Issue: Cache CSS

Reduce the additional overhead of PHP in wiki_css.php by caching the CSS output for wiki CSS to a file. If the file exists, use it instead of calling wiki_css.php. Do not use CSS Roll-up or other platform-based techniques, since these require the file to be pre-generated with preset names. A proper caching mechanism can create roll-up caches from arbitrary lists of CSS and deliver or compile them on the fly.