Extract epic nav into separate partial for easier CE => EE

What does this MR do?

Extract epic nav into separate partial for easier CE => EE

Are there points in the code the reviewer needs to double check?

Shouldn't be

Why was this MR needed?

Reduce conflicts for CE => EE

Screenshots (if relevant)

None

Does this MR meet the acceptance criteria?

• Review
  • Has been reviewed by Frontend
• Backport for CE (https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/15655)

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/4027