Replaces an approval workflow's definition. The whole definition is required — a partial body is
rejected, not merged, and fields this contract does not define are ignored. Send type and
active back as read; changing either here is rejected, and combinations the Rillet app cannot
configure are rejected too (see the individual fields).

Entries already awaiting approval are unaffected, because an entry copies the layers it needs when
approval starts; only entries registered after this call use the new definition. Deadlines and layer
escalation rules are not part of this contract and are cleared, exactly as saving a workflow in
Rillet clears them.