Migrate to Auth0 as Identity Provider

This pull request removes the hard dependency on Google for 
authenticating users and migrates to Auth0 as Identity Provider for OpenDC. 
This has as benefit that we can authenticate users without having to manage 
user data ourselves and do not have a dependency on Google accounts anymore. 

- Frontend cleanup:
   - Use CSS modules everywhere to encapsulate the styling of React components.
   - Perform all communication in the frontend via the REST API (as opposed to WebSockets).
      The original approach was aimed at collaborative editing, but made normal operations 
      harder to implement and debug. If we want to implement collaborative editing in the 
      future, we can expose only a small WebSocket API specifically for collaborative editing.
   - Move to FontAwesome 5 (using the official React libraries)
   - Use Reactstrap where possible. Previously, we mixed raw Bootstrap classes with 
      Reactstrap, which is confusing.
   - Reduce the scope of the Redux state. Some state in the frontend application can be 
      kept locally and does not need to be managed by Redux.
   - Migrate from Create React App (CRA) to Next.js since it allows us to pre-render
      multiple pages as well as opt-in to Server Side Rendering.
   - Remove the Google login and use Auth0 for authentication now.
   - Use Node 16
- Backend cleanup:
   - Remove Socket.IO endpoint from backend, since it is not needed by the frontend 
      anymore. Removing it reduces the attack surface of OpenDC as well as the maintenance efforts.
   - Use Auth0 JWT token for authorizing API accesses
   - Refactor API endpoints to use Flask Restful as opposed to our custom in-house 
      routing logic. Previously, this was needed to support the Socket.IO endpoint, 
      but increases maintenance effort.
   - Expose Swagger UI from API
   - Use Python 3.9 and uwsgi to host Flask application
- Actualize OpenAPI schema and update to version 3.0.

**Breaking API Changes**

* This pull request removes the users collection from the database table. Instead, we now use the user identifier passed by Auth0 to identify the data that belongs to a user.

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