Use `project.fullname` in X-Pagure-Project header
    
    This will make the header include the project's namespace and
    `forks/username` (if it's a fork). Note this could cause some
    procmail filters that use capturing to start putting the mails
    in subdirectories unexpectedly, so should probably be announced.
    
    Fixes https://pagure.io/pagure/issue/1518
    Merges https://pagure.io/pagure/pull-request/1519