Support remote PR and running pagure-ci
    
    Remote PR in pagure have no project_from since they are remote but up
    until now pagure-ci's code was relying on the fullname of the
    project_from. With this commit we add support for remote PR which, when
    trying to trigger a new CI run, can specify a PR UID instead of a project
    fullname. The logic will then retrieve the corresponding PR, check if it
    is a remote PR or not and depend on this use either the project_from or
    the project_to field of the pull-request object.
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr></pingou@pingoured.fr>