Blocker/sub-task/parent task tracking support for Issues

Problem to solve

Using GitLab issues as a replacement for Phabricator Tasks due to migrating from the latter to the former) results in the loss of dependency and relationship information embedded in the Phab tasks. Phab tasks can be marked as parent or child tasks of other tasks, producing a dependency graph like this:

GitLab Issues can be marked as "related" to one another, but there is not a more detailed type of relationship such as parent, child, blocker, etc.

Intended users

Project managers

Further details

Project managers benefit from being able to clearly track development tasks as a required sequence of work items.

Proposal

Allow Issues to be marked as parents, children, blockers, or dependencies of other Issues

Permissions and Security

N/A

Documentation

Would require documentation for the new feature.

Testing

No predicted risk.

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

Success looks like being able to import Phabricator Tasks during migration and have the Tasks' relationships preserved when transformed into GitLab issues. Furthermore, Issues become more powerful tools for developer task tracking because it becomes possible to connect them to one another with more meaningful relationships.