Drafting the new architecture for sidebars

What does this MR do?

In this MR, we start the foundations for the new sidebar system. It will be based on a system of pluggable menus and menu items that will have their own visibility and access rights. That way we can centralize the logic and even reuse menu items later.

The structure is:

• There is only one panel.
• Each panel can have different menus.
  • Each menu can have its own visibility logic, meaning that if the logic is not successful, not menu items will be shown.
• Each menu can have different menu items.
  • Each menu item has its own visibility logic. If the check is not successful, the element won't be displayed.

This MR doesn't replace yet the navbar, it just creates the necessary classes. Now, in following MRs we will add each menu independently.

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have not included a changelog entry because is a tooling change.
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Separation of EE specific content

Related to #326304 (closed)