Follow-up from "Expose epic_iid in issues API"

The following discussions from !15998 (merged) should be addressed:

• @winh started a discussion:

  loading epic eagerly will be done in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/15886

• @winh started a discussion:

  CE-backport: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/32544