[REQUEST] Wire `into_candidates` into release track config

[clemiller]

The API supports a newer release track promotion policy option:

promotion_conflicts: {
  into_candidates
  candidates_to_staged
  staged_to_members
}

The frontend currently exposes conflict policies for candidates_to_staged and staged_to_members, but not into_candidates. into_candidates controls what happens when objects being added into Candidates conflict with an existing version already tracked by the release track.