Allow linking issues to PRs in the initial comment of a PR
    
    Currently, we allow linking issues to PRs in commit messages but there
    is no reason we cannot link issues to PR via the initial comment of a
    PR.
    This commit adds support for this. One can now link an issue with a PR
    in the initial comment, when opening the PR as well as when editing the
    initial comment.
    
    Fixes https://pagure.io/pagure/issue/4309
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>