Frontend: CI Catalog - Add Feature flag and navigation items

This is the very first issue to tackle in the index page. We want to add the navigation elements to the nav bars with the correct amount of visibility behind a feature flag, ci_private_catalog_beta. Clicking on the navigation item takes you to the catalog index page if you have access to the license feature ci_private_catalog (meaning, you have an ultimate license). We show only the haml page with a dummy title for now

This must include feature specs for basic navigation tests.

As a second MR, we could add basic Vue Component and JS file for mounting point + Jest tests for the base component.