Create a new api endpoint for health check

[yunxi-zhang]

At the moment, there's no health check api endpoint available. When deploying with load balancers for health checking, this is not possible.

Request a new endpoint for health checking purpose.

[jcourt562]

You don't have to have another endpoint. We use for instance a GET on / that will return a 404 since the lack of a tails file specifier returns NOT FOUND. This gives proof of life but not proof of working state. That might be enough for you short term until this gets implemented.

Note this is also a duplicate of issue #5