This change removes the socket.io websocket connection/client in favour
of the OpenDC REST API. The socket.io websocket implementation was
intended to be used for interactive and collaborative datacenter design
and exploration.

However, we do not support this functionality at the moment
(collaborative design and exploration) and having the entire API run
over this websocket connection is fragile and not standard practice.

To improve maintainability, we therefore remove the websocket
implementation in favour of the OpenDC REST API implementation using
the fetch API. If we want to implement collaboration in the future, we
will develop appropriate extensions in conjuction with the existing REST
API. For this, we should look for standard and existing implementation
of this functionality.

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.