Health check endpoint, throttles requests, refactors

- Only makes requests to IG once every two seconds (per instance of server)
- `/ig/health_check` returns the following JSON:
```
{
  status_code: 200,
  rate_limit: "5000",
  queries_remaing: "3884",
  current_subscriptions: [
      {
        object: "tag",
        object_id: "nofilter",
        aspect: "media",
        callback_url: "https://17bc7f.ngrok.com/ig/callback",
        type: "subscription",
        id: "14258707"
      }
  ],
  connected_clients: [
      {
        id: "tlPwgWXPzU36JNguAAAA"
      },
      {
        id: "vtCGWWx-lsFH027yAAAB"
      }
  ]
}
```