Bug 1094024 - Implement Coinbase callback receiver

Sure! Let me try a rewrite:

```js
if (!(body && body.order && body.order.status === "completed" && body.order.event.type === "completed")) {
  reply("okay");
}
```

Does it make more sense written in that form? Basically, I want to do an early return if I'm not supposed to handle this event, and the conditions to pass this message into the processing queue are `body.order.status === "completed" && body.order.event.type === "completed"`.