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

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 workaround used to access runtime variables in
the React frontend, in order to improve the ergonimics when using the
development mode of CRA (Create React App).

This change adds support for dynamically specifying various variables
for the frontend Docker image through environmental variables.
Previously, these were embedded directly into the source code, which
requires rebuilding the entire image to change these variables.

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.