Add a healthcheck endpoint for zero-downtime deploys on Render

[floehopper]

See https://docs.render.com/docs/deploys#zero-downtime-deploys

[floehopper]

I added a healthcheck endpoint in c4f63fe and configured it in Render triggering this deployment.

The healthcheck endpoint is at https://jam.coop/up and since it's based on the built-in Rails endpoint, it displays a green or red page as well as returning an appropriate HTTP status.

At the moment it only checks that the Rails app and the connection to the db are working. We might want to extend it to check other things like Sidekiq, Postmark, etc.