Promote issue to epic - System notes for closing issue and new epic

Follow up of https://gitlab.com/gitlab-org/gitlab-ee/issues/3777.

• So that there is a system note on the closed issue. (Similar to when you move an issue, and the note on the closed issue.)
• So that there is a system note on the new epic. (Similar to when you move an issue, and the note on the new issue.)