Make frontend configurable

Related to #1; for running locally in development, it will be okay to have the backend API endpoints hard-coded as it is now, in the frontend `TodoLists` component, but to be able to run elsewhere (#7) it needs to be a bit more dynamic. Maybe the backend base URL should be `/api/...` when built and deployed somewhere, and `http://localhost:3001/api/...` when running in development.