Add Health Check Endpoint

[JamesFrost]

Having a dedicated endpoint that could be used to check the health of the service would help enable monitoring PagerBeauty deployments. This is a common pattern that can be used with other services.

We're currently using the /v1/schedules endpoint for health checking, but this isn't ideal, as it isn't the purpose of this endpoint. Additionally, this endpoint displays information about the schedules, and so should be authenticated.

Optionally, the health check endpoint could do some functional checks e.g. is the PagerDuty integration still working.