no issue
- moves the `NavItem` object from the navigation controller to an explicit `NavigationItem` model file
- adds a custom transform `navigation-settings` that transforms the navigation settings JSON string to/from an array of `NavigationItem` objects
- simplifies the `settings/navigation` controller as it no longer has to export it's own internal model and handle serialization and deserialization
This pattern should also help simplify the apps/slack integration code if implemented there.
