Tools for manual grouping of package pull requests into a project pull request

We currently have capability to merge forwarded project PRs and remove package pull requests from them.
There's nothing that would add an arbitrary package PR to a project PR.

`git-obs pr create` should have `--target=owner/repo#number`
`git-obs staging add --target=owner/repo#number <package_pr> [package_pr] ...`

Implementation details:
- check that the target is really a project