Add health check endpoint

[Mr0grog]

To support various tools (and in particular, Kubernetes), we should add an HTTP health check endpoint to the server. It doesn’t technically matter what it returns (but we should be consistent and return some JSON) so long as it is able to quickly return a 200 status code.

Other tools and use this to determine whether the server is healthy (and, if they have the rights, optionally kill and restart it).

I’m thinking it should be at GET /healthcheck.

See also edgi-govdata-archiving/web-monitoring-processing#154.