Add a private column to the pull_requests table
    
    This makes the PR object closer to the Issue one allowing to re-share
    more code and having private PRs is something that has already been
    asked (cf ticket #1893) for pagure so this is laying ground for it.
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr></pingou@pingoured.fr>