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 This prevents our cache getting out of date with the file system.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

usr/lib/python2*/*-packages/cubicweb/server/
usr/lib/python2*/*-packages/cubicweb/hooks/
usr/lib/python2*/*-packages/cubicweb/sobjects/
usr/lib/python2*/*-packages/cubicweb/schemas/
usr/share/cubicweb/migration/