Update mailman3 nginx configuration
    
    This imports some changes which are present in the lists.o.o/mailman3
    nginx configuration since some time without having been reflected in
    Salt. I don't know why these changes have been made, but the service
    seems fine with them.
    
    Signed-off-by: Georg Pfuetzenreuter <georg.pfuetzenreuter@suse.com>