Add a global configuration option to turn on or off fedmsg notifications
    
    This configuration key allow to turn on or off fedmsg notifications on the
    entire instance, not just for one project.
    
    Fixes https://pagure.io/pagure/issue/2717
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr></pingou@pingoured.fr>