[etwist] remove no-longer-necessary uicache cleaning It was done before we were actually up, which means it could be too eager when combined with #6893185. Closes #6893247.

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 This prevents our cache getting out of date with the file system.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

[test] Add missing u'' Test actually works in py3k, but not in python 2... That has to be a first!