cdn-common: set default expiry to avoid caching redirects for too long
cdn-common: set default expiry to avoid caching redirects for too long

While we are no longer permanent redirects for most things, we were
still telling browsers to cache the redirect for 1 week, which caused
infinite redirection loops.

libravatar/libravatar,libravatar/libravatar,libravatar/libravatar,libravatar/libravatar,libravatar/libravatar,libravatar/libravatar,libravatar/libravatar