View roadmap in epic

internal customer since GitLabbers are using child epics and want to see work breakdown structure of epics in a tree view.

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[Frontend Issue]

• View a roadmap of children epics of the current epic.
• This should be in a separate roadmap tab.
• The existing description area and the child epics and child issues area below the description are unchanged.

Original description

Designs

Discussion tab	Tree tab	Roadmap tab

• First tab is discussion - comments, threads, and system notes
• Second tab is the tree view. All epics and issues nested under the current epic should be viewable, collapsible, and draggable (see https://gitlab.com/gitlab-org/gitlab-ee/issues/7327#note_151840087)
  • Dropdown on tree view allows you to view Entire tree, Epics only, and Issues only
  • Removing an epic also removes all of its nested epics from the current epic only (it doesn't break existing relationships between epics)
  • Dialog to confirm removal
• Third tab is roadmap- current epic should show, along with every nested epic
  • There's limited space, so we'll have to show fewer quarters/months/weeks than the regular roadmap view
  • There might end up being a button to go directly to Roadmap

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