[web] remove PropertySheet._cache Instead of remembering that a css file has been processed, we re-process it at every request. This prevents our cache getting out of date with the file system. Closes #6893185 While we're at it, write the file atomically so that we don't risk serving partial data.

version=3
opts=uversionmangle=s/(rc|a|b|c)/~$1/ \
http://pypi.debian.net/cubicweb/cubicweb-(.+)\.(?:zip|tgz|tbz|txz|(?:tar\.(?:gz|bz2|xz)))