This change updates the structure of the OpenDC frontend in order to
improve the maintainability of the frontend.

This change updates the frontend so that the modal state is not stored
inside Redux but instead is stored using the useState hook. This
simplifies the design of the modal components.

This change updates the web frontend to use Next.js instead of Create
React App (CRA). Next.js enables the possibility of rendering pages on
the server side (which reduces the time to first frame) and overall
provides a better development experience.

Future commits will try to futher optimize the implementation for
Next.js.

This change updates the Redux store initialization to ensure that the
Redux logger is last in the middleware change. If we do not do this,
Redux Logger might log thunds and promises, but not actual actions.

See https://github.com/LogRocket/redux-logger/issues/20

This change updates the project structure to become flattened.
Previously, the simulator, frontend and API each lived into their own
directory.

With this change, all modules of the project live in the top-level
directory of the repository. This should improve discoverability of
modules of the project.