FE: Display hierarchy widget for work items with type Epic

Summary

When visiting a work item with the Epic type we should display the hierarchy widget to list child epics and issues, similarly to the objective page.

The Add dropdown needs to include the following options:

• New epic, should create and link a new work items with type Epic
• New issue, should create and link a new work item with the type Issue
• Existing issue, should display the autocomplete field that fetches work items with the Issue type (we currently use project_work_items.query.graphql for this). These results will already include legacy issues, so selecting one will be sufficient to add it as a child.
• Existing epic, should display the autocomplete field that fetches work items with the Epic type
• When child items are selected, they should be opened in a full page view (like OKRs) not a modal view (like tasks)