Move project sidebar refactor to 'lib' folder

What does this MR do?

In the Project sidebar refactor, we're creating menus and menu items in a structured way with classes or models. At first, these classes were created inside the app/models folder because they were using concerns and also they were rendered directly by the views.

Nevertheless, and after several discussions, it seems it fits better in the lib folder, since it's not totally an ActiveRecord model.

In this MR we're moving all the related classes from app/models to the lib folder.

Does this MR meet the acceptance criteria?

Conformity

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