Add support for fedora-messaging in pagure
    
    This effectively port pagure to the new messaging app used in Fedora
    relying on AMQP.
    As for the other messaging notifications this relies on a dedicated
    configuration key.
    
    However, projects will reuse the "fedmsg" option key.
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>