ChangeTransferPolicy controller should merge autonomously

[crenshaw-dev]

Right now the PromotionStrategy controller enforces its ordering rules by directly managing the PullRequest under a given ChangeTransferPolicy.

Instead, the PromotionStrategy should inject something like an previous-environment check in environments besides the first one, and the ChangeTransferPolicy controller should independently merge PRs once the checks pass.

This frees the PromotionStrategy controller of having to manage promotion logic directly and allows it to delegate that work via the existing mechanism. It also allows us to replace the vanity "copied statuses" with an actually-used status to gate promotion.