Allow viewing a PR when its origin (fork or branch) is gone
    
    This will ensure a pull-request remains accessible despite its source
    being gone by relying on the newly introduced reference for each PR.
    Since that reference is in the main repo and is not going away when the
    fork or branch gets deleted, it is just as easy to rely on it when the
    source of the PR disappears.
    
    Fixes https://pagure.io/pagure/issue/2492
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>