Add a healthcheck endpoint to the API

[ian-noaa]

Problem

During work on #104, the application failed to launch because the required environment variables (in particular FLASK_DIAG_DIR) weren't set. This wasn't detected as the application won't warn or fail until it attempts to use the environment variables to access the resources. This resulted in a day of debugging the production cluster

Solution

We should have a /health endpoint that checks for required config and tests to make sure external resources (s3, databases, etc...) are available.

No Gos

Describe any features or behaviors that have been considered and rejected as out of scope for this project.

Rabbit Holes

Describe any solutions to problems that pose a risk to completing this project on time.