[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 This prevents our cache getting out of date with the file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.
include *.py
include */*.py
recursive-include data *.gif *.png *.ico *.css *.js
recursive-include i18n *.po
recursive-include wdoc *