Implement 'healthz' endpoint for health checks

[jpds]

Issue

Impacted version: v0.50

Server Setup Information:

• Deployment Method(tar/sandstorm/snap/docker/etc /http frontend (nginx, httpd, ...)): Docker with nginx

Problem description:

It'd be great if wekan had a /healthz endpoint which could be used by tools such as Kubernetes for health checks:

• https://www.ianlewis.org/en/using-kubernetes-health-checks
• http://kubernetesbyexample.com/healthz/