This makes a few updates to health checks, to ease usage outside of our

own Docker setup:


The endpoint has been renamed from /_health/ to /health/, making

   it clear it's meant to be used outside of Review Board's own code.


The list of accessible IPs now defaults to ['127.0.0.1', '::1'],

   as opposed to the INTERNAL_IPS default of [].


A HEALTHCHECK_IPS setting has been added for custom IPs. That

   combines with the defaults and is used to generate

DJBLETS_HEALTHCHECK_IPS. A default for this is in the generated

conf/settings_local.py.


The Docker image has been updated to use the new health check endpoint.
New documentation has been written to help users configure monitoring of

the health check endpoint and to customize access.

Tested the Docker image, temporarily shutting down the cache and

database servers and monitoring the health status.
Tested adding my IP to the list of internal IPs and verifying output.
Built the docs. Checked for build errors, spelling errors, and bad links.