Add a simple /healthz path on the Nginx frontend

[alexmbird]

This adds a simple, unauthenticated /healthz path which can be used to monitor the availability of a Streisand box using a service like uptime robot. It returns a HTTP 200 message with body ok. The body is intentionally nondescript so as not to expose the fact that the host runs Streisand to unauthenticated visitors.

[nopdotcom]

I think this would have to be optional. A lot of people are paranoid.

Have you considered using https://streisand.example/robots.txt, delivering this file?

User-agent: *
Disallow: /

[alexmbird]

@nopdotcom I've spruced up this PR so that:

1. It serves a robots.txt (which is a great idea, it's more standard than a /healthz and just as useful)
2. Makes that optional with a default of no. I've rolled out a couple of Streisand instances to test this works as expected.

Let me know if you'd like to see any other changes.