Chore: Update react-scripts to v4

[tuliomir]

This application management is linked to the react-scripts dependency. It's necessary to update it before the others to ensure all of the latest versions of all dependencies are as inter-compatible as possible.

It was not possible to upgrade all the way to the latest version 5.0.1 because of issues described on #441 , so the upgrade finished on 4.0.3.

Acceptance Criteria

• React Scripts should be at the latest v4 stable version: 4.0.3
• ESLint should be also upgraded to match the internal versions from react-scripts
• Unit tests should be adapted to changes in the updated Jest behavior

Security Checklist

• Make sure you do not include new dependencies in the project unless strictly necessary and do not include dev-dependencies as production ones. More dependencies increase the possibility of one of them being hijacked and affecting us.