[REF] *: BS4, review the 'nav' and 'navbar' components

Now the nav structure is more declarative: the 'nav-item' and 'nav-link'
classes have to be put on related nav item and links. Also, the 'active'
class has to be put on links instead of items.

Same as for the navbar component:
- The navbar structure was simplified and meant to be more declarative.
- The navbar colors are also now handled by a different set of classes.

The 'nav-stacked' class was removed and replaced by flex utilities.

The set of classes for colors and nav styles also changed.