Pluralize route API paths

During the specification period, it was suggested that pluralized object names
are more idiomatic in APIs than singular. This changeset simply adopts that
preference for the API going forward. The client has been updated to remain
compatible.
Implement V2 API base endpoint

This implements a base endpoint that will respond with a 200 OK and an empty
json response. Such an endpoint can be used as to ping the v2 service or as an
endpoint to check authorization status.