Removing inheritance in menu items sidebar refactor

What does this MR do?

The team gathered to discuss the implementation of the refactor in order to reuse it in other places. The conclusions were:

• Remove the inheritance for menu items. The top parent menu will build them and provide all the necessary info it needs. It will also take care of checking whether the item can be displayed or not (based on policies)
• Move the menus outside of the folder. At the moment each menu lives in sidebars/projects/menus/menu_name/menu.rb and we're going to move it one folder up into sidebars/projects/menus/menu_name_menu.rb.

All the generated HTML structure remains the same.

Does this MR meet the acceptance criteria?

Conformity

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