Add migration to fix the project_from_id foreign key in pull_requests
    
    This way, if the project the PR originates from is deleted, the PR
    remains and remains open.
    
    Fixes https://pagure.io/pagure/issue/2729
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr></pingou@pingoured.fr>