Add hierarchy depth to roadmaps

Problem to solve

As a User of Roadmaps,

I need to be able to see the sub-epics under an Epic,

So I can get a detailed, view of the work involved over a timeline

• In the roadmap view, you can currently view epics and their direct child epic (with work in !23600 (merged))
• But to view more than one layer/descendent down, you have to click into a child child with the roadmap to view its children.
• The child epics under an epic should nested below it with a depth of five, similar to the epic tree.
• If an epic is a child epic and appears under a parent epic, it should not be repeated anywhere else in the roadmap.

The roadmap needs to support different views, for different use cases within a a scaling business.

• Team/Group type roadmap where work in progress is tracked/reported on and upcoming work is sequenced
• Release/Milestone Roadmap where work in progress for a set timeline is tracked/reported on and upcoming work is sequenced
• Dept/Program level reporting where you look at the aggregate work of multiple teams together
• Portfolio Roadmap where Im tracking big rock product work across multiple programs/depts/products
• Executive/Leadership view where top level business stakeholders have a "5000 ft view" of big ticket work in progress at an org against a timeline

Example:

• Epic 1
  • Epic 2 (Child of 1)
    • Epic 3 (Child of 2)

1. If filtering only matches Epic 1, only return Epic 1
2. If the filtering only matches Epic 2, only return Epic 2
3. If the filtering matches Epic 1 AND 2, return 1 with 2 nested under it.
4. If the filtering matches Epic 2 AND 3, return 2 with 3 nested under it.

Acceptance Criteria

• Allow the user to view all child epics within an epic, up to the depth of 5 similar to the epic tree. Limit is handled on the backend, so no need to explicitly handle on the frontend
• Epics should only show up once. Children currently show up as a child and as a top level epic

• Specs: https://gitlab-org.gitlab.io/gitlab-design/hosted/alexis/%237077-expand-epics-in-roadmap-v2-spec-previews/

Interested Customers/Prospects

• Interested Prospect
• Interested Prospect
• Interested Customer
• Interested Prospect

Links / references

• Previous description notes

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