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