This change is a rewrite of the existing OpenDC frontend in order to
migrate to the PatternFly 4 design framework.

PatternFly is used by Red Hat for various computing related services such
as OpenShift, Red Hat Virtualization and Cockpit. Since their design
requirements are very similar to those of OpenDC (modeling computing
services), migrating to PatternFly 4 allows us to re-use design choices
from these services.

See https://www.patternfly.org/v4/ for more information about
PatternFly.

This change updates the OpenDC frontend to use React Query for fetching
and mutating project data. Previously, this state was tracked and
synchronized via Redux. Migrating to React Query greatly simplifies the
state synchronization logic necessary in the frontend.

This change removes the current active identifiers from the Redux state.
Instead, we use the router query to track the active project, portfolio
and topology.

This change updates the frontend to use the FontAwesome 5 React library
that renders SVG icons as opposed to CSS icon fonts. This migration
resolves a couple of issues we had with server-side rendering of the
previous FontAwesome icons.

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 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.