[Part 5] Add support for Parent Epic in Epic sidebar

What does this MR do?

This MR adds Parent Epic support in Epic sidebar within refactored Epics app which is being developed as a part of &410 (closed) (and is currently being Cookie flag that can be turned on by running document.cookie = "load_new_epic_app=true" in browser console).

Does this MR meet the acceptance criteria?

• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?
• Security reports checked/validated by reviewer

#7751 (closed)