Relax progress.o.o rate limit after challenge
    
    As a POW challenge was introduced, the rather restrictive rate limit can
    be simplified. Only apply the strict limit for expensive paths to
    non-validated clients (which can, as per the challenge rules, only apply
    to crawlers we exempted from being challenged) and let validated clients
    browse freely again.
    
    Signed-off-by: Georg Pfuetzenreuter <mail@georg-pfuetzenreuter.net>