Make related issues child issues when promoting an issue to an epic
Proposal
When promoting an issue to an epic, make related issues of the original issue, child issues of the new epic.
Concerns
- Some issues of the original issue may not be "valid" child issues of the new epic. We might want to loosen this validity criteria, per https://gitlab.com/gitlab-org/gitlab-ee/issues/6750#note_85271700, to make it more flexible, but then would put more responsibility on users to use epics and issue relationships in general more carefully.
- Currently, an issue can only have one parent epic. If we have this automation happen, we might take an issue and assign it a new parent, which may not be what the user wants, especially since related issues are very loose relationships.
- The user may simply not want a related issue part of the new epic, because it logically doesn't make sense.
Proposal amendment to address concerns
- After the promotion, there is an additional UI step where GitLab asks you to choose which issues you (from the previous related issues list), you'd like to make child issues of the new epic. So you can pick and choose (with simple clicks), and make that commitment then and there.