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.

This change splits the Docker Compose configuration into three files in
order to share the configuration for different environments (e.g.
development and production).

Furthermore, this change drops the requirement for a reverse proxy.

This change fixes an issue where we did not forward the environmental
variable for controlling the API url to the React build.

This change makes the frontend independent of the API by removing the
static file serving logic from the API server. Instead, we can serve the
frontend as static HTML over CDNs.

This change prepares the frontend Git repository for the monorepo
residing at https://github.com/atlarge-research.com/opendc. To
accomodate for this, we move all files into a frontend subdirectory.