With the previous setup, browsers were free to use a local cache for any
length of time they pleased: we didn't set an 'Expires' header (or max-age
directive), and Cache-Control just said "you're free to cache this".
Therefore be more explicit: we don't mind if browsers cache things, but they
MUST revalidate everything on every request.
GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3221