[BB-2220] Add navigation customization page

Created by: Agrendalath

Styling of the right side of the navbar needs some tweaks.

This PR adds button customization page to the new Ocim frontend.

Testing instructions:

1. Checkout this branch.
2. Run Ocim (inside vagrant with make run.dev).
3. Go to frontend/ and build the new API client with ./scripts/build-api-client.sh
4. Run the new frontend: npm start.
5. Create a new test instance and log in the console.
6. Go to the Theme -> Navigation page and make sure it's working as expected (http://localhost:3000/console/theming/navigation).
7. Change colors and check if the values are correctly saved in the backend.

Reviewers:

• @giovannicimolin