views: Add a /health healthcheck endpoint.

This endpoint verifies that the services that Zulip needs to function
are running, and Django can talk to them.  It is designed to be used
as a readiness probe[^1] for Zulip, either by Kubernetes, or some other
reverse-proxy load-balancer in front of Zulip.  Because of this, it
limits access to only localhost and the IP addresses of configured
reverse proxies.

Tests are limited because we cannot stop running services (which would
impact other concurrent tests) and there would be extremely limited
utility to mocking the very specific methods we're calling to raising
the exceptions that we're looking for.

[^1]: https://kubernetes.io/docs/tasks/configure-pod-container/configure-liveness-readiness-startup-probes/

zproject/urls.py

@@ -54,6 +54,7 @@ from zerver.views.documentation import IntegrationView, MarkdownDirectoryView, i
 from zerver.views.drafts import create_drafts, delete_draft, edit_draft, fetch_drafts
 from zerver.views.email_mirror import email_mirror_message
 from zerver.views.events_register import events_register_backend
+from zerver.views.health import health
 from zerver.views.home import accounts_accept_terms, desktop_home, home
 from zerver.views.hotspots import mark_hotspot_as_read
 from zerver.views.invite import (
@@ -836,6 +837,9 @@ urls += [
     path("api/v1/", include(v1_api_mobile_patterns)),
 ]
 
+# Healthcheck URL
+urls += [path("health", health)]
+
 # The sequence is important; if i18n URLs don't come first then
 # reverse URL mapping points to i18n URLs which causes the frontend
 # tests to fail