Create Proposal Test Environment

Open a private temporary environment holding this proposal's changes, forked from current Production, so a reviewer can ask Dot before deciding. The proposal still merges into Production. Returns the reviewer's existing one when it exists, first merging in what the proposal and Production gained since (refreshed). One that cannot take those changes answers 409 test_environment_outdated, since it may hold the reviewer's own work: with replace a new one is opened (created) and the old one stays as it is. When a new one could not take them either, it answers 409 proposal_conflicts (the proposal conflicts with Production) or 503 (Production has a write waiting for its commit).

Body: replace: Optional. Open a new environment when the reviewer's own cannot be updated.

Returns: {"environment": {id, slug, name, color, ephemeral, created_by, exists}, "created": bool, "refreshed": bool}