Is your feature request related to a problem? Please describe.

Per #2073 and #2088, we're making Menu, User, and Comment constructors protected.

For more context, see the linked issues.

Acceptance Criteria

• Implement static Timber::get_menu() method which in turn call the appropriate factory methods (factory implementations themselves are out of scope here)
• Ditto Timber::get_user()
• Ditto Timber::get_comment()
• Implement Timber::get_users() methods for getting collections.
• Ditto Timber::get_comments()
• Determine whether we need a ::get_menus() method? (No - see discussion.)
• Determine which hooks, if any, we need to apply before instantiating Menu or User objects (should they get their own class maps?)
• Implement Class Maps for MenuItem
• Implement tests for these methods.

Describe alternatives you’ve considered

The alternative is having an inconsistent API, which I personally don't find acceptable.

Additional context

See #1793 for how this all got started.