Spike: Child epics from a different hierarchy

Requirements

• From within an epic, paste a URL for an epic in another group that you have access to make it a child.
• We shouldn't worry about searching/autocomplete. There could be performance concerns with this workflow and customer feedback is that this is not their preferred path to create parent/child relationships.
• The epic tree should redact epics,subepics and issues that a user does not have access to.
• In ancestors in the sidebar for epics, the parent/ancestor epic should be redacted if the user does not have access to that epic's group.
• In the roadmap, the epic name should be redacted if the user does not have access to that epic's group. It should be included in the rollups for item count and % complete.

Permissions

Action	Permissions
Add a child epic	The user needs admin_epic permissions for the parent epic and the child epic. The feature subepics should be available for the parent epic's group and the child epic's group.
Reorder child epics in the epics tree	same above
Remove child epics from the epics tree	same above
List child epics	The user needs read_epic permissions for the parent epic and the child epic
List ancestor epics	same as above

Tasks

• https://gitlab.com/gitlab-org/gitlab/-/issues/373713+
• Add a limit on number of child epics per epic (#373700 - closed)
• Allow adding child epics from different group h... (#373704 - closed)
• Allow listing ancestor epics from different gro... (#373708 - closed)
• Allow listing child epics from different group ... (#373709 - closed)
• Redact ancestor epics in sidebar (#373711 - closed)