Add frontend support for Relationships between Epics in refactored Epics app

What does this MR do?

This MR ports over changes introduced in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/8637 to refactored Epics app (currently behind cookie flag, which can be enabled by running document.cookie = "load_new_epic_app=true") to have feature parity.

Please note that refactored Epics app is already in master (from https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/8059), this MR just upgrades it with new feature added in older app.

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
• Conforms to the database 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

#8333 (closed) #7750 (closed)