Runtime environment parameters

The frontend build should support the runtime configuration of the environment variables defined in the .env file.
This is required so that a Docker image of the frontend (see #141 (closed)) can be used with various configurations, without the need to rebuild the image.

As a possible solution, the react-inject-env package can come to help, which provides the capability to create and edit an env.js file the build folder. Environment variables defined there are loaded by the React application.

Note: the PUBLIC_URL cannot be made dynamically configurable that way, as it is already used in the index.html, before loading the env.js file.