Approve a batch

Sign the batch and, once it has every signature, check the business can cover the run. A batch awaiting approval is signed through the approvals engine, the same rule as `POST /approvals/{id}/approve` (IN-362): the uploader is refused, and a policy needing more signatures leaves the batch awaiting approval, which the response's `status` says. Only a role that may decide approvals signs; anyone else is told the batch is still waiting.

curl -X POST "https://example.com/api/b/v1/payout-batches/497f6eca-6276-4993-bfeb-53cbbbba6f08/approve"

{  "id": "497f6eca-6276-4993-bfeb-53cbbbba6f08",  "name": "string",  "source": "string",  "currency": "string",  "status": "validating",  "total_lines": 0,  "valid_lines": 0,  "invalid_lines": 0,  "total_minor": 0,  "approval_id": "15af2260-72db-4fc4-b587-6b841d6a3d55",  "created_at": "2019-08-24T14:15:22Z",  "completed_at": "2019-08-24T14:15:22Z"}