[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 This prevents our cache getting out of date with the file system.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.

../../doc/book usr/share/doc/cubicweb-documentation
../../doc/_build/html usr/share/doc/cubicweb-documentation