Keep related issues when promoting to epic

Problem to solve

An issues that is promoted to an epic may have related issues that are no longer visible on the epic.

Intended users

Everybody promoting issues to epics and using related issues.

Further details

Promoting issues to epics was introduced in https://gitlab.com/gitlab-org/gitlab-ee/issues/3777.

Proposal

Automatically add related issues to the epic:

Permissions and Security

Problem to solve:

The person promoting the issue to an epic may not have permission to see all related issues.

Documentation

Testing

What does success look like, and how can we measure that?

What is the type of buyer?

Links / references