Add simple 'ping' endpoint for healthchecks

Add simple 'ping' endpoint for healthchecks

To determine if the server is online, we want to be able to 'ping' it
with as little configuration and access possibilities as possible. The
only available option for this today, is anonymous access.

Let us thus add a new endpoint, that replies with 'PONG' when queried.

This will need anonymous access enabled, but can be restricted to:
a) localhost
b) without any access permissions

Signed-off-by: Olliver Schinagl <oliver@schinagl.nl>