Add readiness probe

[RealOrangeOne]

What is the context of this PR?

https://jira.ons.gov.uk/browse/CMS-215

When running in Kubernetes, it needs a way to identify when the application is "ready" to serve requests. This is done using a readiness probe.

There are other probes available, which I've not implemented:

• "Liveness" probe probably isn't useful to us right now. Application processes are cycled regularly, so shouldn't result in deadlocks. Monitoring external infrastructure here (eg database connectivity) could result in death spirals on external issues.
• "Startup" probe may potentially be useful to allow the application to be considered "ready" to serve requests sooner, but since the behaviour would be the same as this readiness probe, it can use the same endpoint.

How to review

There are no visible changes, just a new endpoint.

The endpoint has intentionally been designed to be as lightweight as possible. No database queries, no templates, just a functional view.

I've added a set of "internal" URLs under the /-/ prefix. The intention is that these URLs are not externally accessible, and exist only for internal use (monitoring, readiness probes etc). In future, these will be blocked from public use by the infrastructure (somewhere).

Follow-up Actions

Configuring the endpoint to be used is out of scope of this PR.

[sanjeevz3009]

Reviewed!