Fix liveness and startup probes

[LeoColomb]

Follow up of #244 This PR should improve the use of Kubernetes probes.

• Nginx status checking if the pod is still live (liveness)
• /login response checking if the app is still responsive (readiness, unchanged)
• /login response checking if the app startup is done (startup)
  • Should help to take care of migrations at startup
  • Follow up of https://github.com/netbox-community/netbox-docker/pull/1203 & https://github.com/netbox-community/netbox-docker/pull/1233