Implement special GitLab markdown reference for milestones

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/3453 by @eReGeBe, with two additions:

- Move changelog item to 8.8
- Fix cross-project milestone ref with invalid project, like https://gitlab.com/gitlab-org/gitlab-ce/commit/f7348cd348ad8f4a18d74dd668283a4e236f5790 did for labels

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/13829

See merge request !3897