Set up DO deployment health checks / downtime detection for front-end

[slifty]

The admin interface went down due to a misconfiguration -- we didn't get notified of this problem but rather noticed it.

While CI / CD won't be able to detect this a priori, we should somehow have a health check (either on the DO side of things, or via uptime robot)

[slifty]

To clarify: if DO provides a "pre-launch-health-check" function we should use that instead, ideally a broken build would never reach a user for any amount of time.