Default to using port 3000 for backend, otherwise prompt user

[brylie]

Our default deployment scenario is to deploy both the client and backend code to the same server. The client will be served via the standard HTTP port(s) while the backend will use port 3000 by default.

We don't want the end-user to think about ports or sub-domains unless necessary.

By default then, the client code should attempt to connect to a backend on the same server address at port 3000. Only in the case where the client cannot connect to a backend on the same server at port 3000 should the user be prompted for connection details.