Use project name as subject tag in notifications
    
    When a project name is avaialble set that project name in the subject
    instead of the non-descript 'Pagure', this is done via an optional
    parameter on the send_email() function.
    For non-project-releated emails that previous behavior is preserved.
    
    Also adds a X-pagure-project header.
    
    Signed-off-by: Simo Sorce <simo@redhat.com>