Add a Health Endpoint

[tpendragon]

We want monitoring to tell us when things are down, not Honeybadger exceptions.

See https://github.com/pulibrary/pdc_discovery/pull/527/files

Things to monitor:

• [ ] Postgres
• [ ] Redis
• [ ] Solr

[tpendragon]

Add a Datadog check when the health endpoint is deployed to prod.