Create the /status endpoint

This implements a status endpoint that displays how things are doing.

Example response:

```json
{
  "status": "ok",
  "sidekiq": {
    "status": "REACHABLE",
    "jobs": 0
  },
  "redis": {
    "status": "REACHABLE"
  }
}
```

Note:

- adds minitest-focus support
- adds sinatra reloader for development :sun_with_face: 

Trello card: https://trello.com/c/4LSZ9dlf