Do not set the Reply-To and Mail-Followup-To headers when not needed
    
    Basically, when there is no milter configured, you can't reply to a
    ticket or PR by email. So in those cases, don't set these two headers at
    all, they are more confusing than anything...
    
    Fixes https://pagure.io/pagure/issue/3399
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>