Convert command server to GraphQL endpoint

[cowboyd]

Rather than implement our own command interpreter we'll get one for free using GraphQL. This will allow us to not only get a command a query language (and response payload) with little effort, but we'll be able to use all of the tooling surrounding the GraphQL ecosystem (like the interactive webshell)

Right now, the command server doesn't do anything except echo the text "Your wish is my command" to any response. Instead, it should be a full GraphQL endpoint. The GraphQL endpoint should only handle responses of type application/json so that later we can serve the web UI over text/html

The goal here is to get the GraphQL interpreter in place, not to actually do anything with it yet, so we'll define and implement minimal schema that just contains an echo query that can be used to verify that the http mechanics are up and running:

type Query {
  echo(text: string!): string;
}

thus
query: { echo(text: "hello world") }
response: { "echo": "hello world" }

[cowboyd]

This is done.