Added a /healthcheck endpoint for load balancers

Added a public /healthcheck endpoint which returns a 200 status code. This is used to let load balancers know that the node app is ready to receive requests.

Use case:
The kibana proxy app is deployed to EC2 behind a load balancer
The load balancer performs a health check by hitting a specified Url
If Google OAuth is enabled, the load balancer will receive a 302 status code
The load balancer thinks the app in not healthy because it did not get a 200 status code
The app will not be registered in the load balancer and will not receive traffic