[LEBV-10] Add header bar
🎯 Summary
This MR adds the header bar to the UI with its final design, including a working language selector.
This MR additionally add some pending technical stuff:
- Fix of existing unit tests.
- Add a proper (and very simple) README.
- Lighthouse HTML reports now are published in Jenkins.
- More reports in Jenkins: tests result, test coverage and linter results.
🧠 Considerations
N/A
🧐 Checklist
- Changes work in Google Chrome
- Changes work in Safari
- UI is responsive