Environment dependent configuration

- express app is configured depending on the current environment, which
  is set via the `NODE_ENV` environment variable
- env configuration is stored in one JSON file per env: `config-dev.json`
  and `config-prod.json`
- an additional `config-local.json` file may be used to override config
  locally (this file is never versionned)
- a new `GET /config.json` route has been added which serves the configuration
  as JSON depending on the current environment used by the server
- the frontend app consumes this object for configuring itself
- added Make rules for running server in dev mode
- updated README

**Edit:**

- moved the JSON configuration files to a new `config/` directory

r=@tOkeshu and every people involved in related discussions