Refactor: Move extensions_gallery_view_model into Settings railway

Issue: [Refactor] Move Web IDE extensions_gallery_view... (#495800 - closed)

What does this MR do and why?

This MR resolves #495800 (closed) by refactoring some business logic out of extensions_marketplace.rb and into the official settings railway.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

There should be no changes. Everything still works 🎉

User preferences enabled extensions marketplace	User preferences disabled

How to set up and validate locally

1. Make sure web_ide_extensions_marketplace feature flag is enabled.
2. Start GDK.
3. Visit Web IDE for a project and go to Extensions in the sidebar. Does this work? If you enable/disable from user preferences, does it work?