Added system settings to enable customization of the top bar navigation (main/user)

### What

Considering 2.3 introduced some "containers" to pull top menu items from, this commit allow customization of those containers (using `main_nav_parent` & `user_nav_parent` settings).

This brings the ability to build different top menus, which could be defined in user/group settings. That way, we can tailor custom UX according to the kind of user using the manager (content editor, developers...)

### To do before merge

- [ ] create appropriate lexicon strings