Mirror GitHub pull requests from forks

Suggested in https://news.ycombinator.com/item?id=12332097

When you move your open source project to GitLab you probably want to keep the repository open.

But you can't disable pull request so you will get pull requests on GitHub while you really want merge requests on GitHub.

It would be nice to automatically import them and close them with a reference to the GitLab location. One problem is that they are from forks. We have a couple of options:

1. Allow an MR from an external location, like federated merge requests https://gitlab.com/gitlab-org/gitlab-ce/issues/4013
2. Mirror the fork
3. Add the branch from the fork to the repository

Related: import cross repo pull requests correctly https://gitlab.com/gitlab-org/gitlab-ce/issues/20153