First stab at a health API

:eyes: 

#357 introduced a super-duper simple health API with one metric. This PR is a humble attempt to satisfy some of the requirements on this [Trello card]. This is what the output looks like right now:

```json
{
  "redis": {
    "used_memory_peak": 1107616,
    "used_memory": 1041200,
    "connected_clients": 3,
    "uptime_in_seconds": 87886
  },
  "postgresql": {
    "connections": 3,
    "waiting_on_lock": 0
  },
  "sidekiq": {
    "active_workers": 0,
    "total_failed": 0,
    "total_processed": 7,
    "dead_jobs": 0,
    "retryable_jobs": 0,
    "scheduled_jobs": 1,
    "queued_jobs": 0,
    "latency": 0
  },
  "supermarket": {
    "expired_ocid_tokens": 1
  }
}
```

@cwebberOps is this remotely close to what you'd envisioned? Where do we go from here?