Add a health check

As well as the usual function of a `/healthcheck` endpoint – to return a `200` response whenever the app is up and running – this adds an application-level check that the scheduled publishing queue has the right number of items in it. We should be able to add a monitoring check for this, and any other checks we decide to add, without having to couple the monitoring stack to our application logic.

Example output when everything’s ok:

```json
{
    "checks": {
        "schedule_queue": {
            "status": "ok"
        }
    },
    "status": "ok"
}
```

When it isn’t:

```json
{
    "checks": {
        "schedule_queue": {
            "message": "3 scheduled editions; 2 items queued",
            "status": "warning"
        }
    },
    "status": "warning"
}
```