[Refactor] Move Web IDE extensions_gallery_view_model into Railway

MR: Refactor: Move extensions_gallery_view_model in... (!167495 - merged)

Description

As identified in this comment, let's move some custom logic in extensions_marketplace.rb into the settings railway.

Acceptance Criteria

• The ExtensionsMarketplace.webide_extensions_gallery_settings only calls the Settings railway.
• The ExtensionsMarketplace.webide_extensions_gallery_settings method is renamed to webide_extensions_gallery_view_model. Not worth it, especially since we want to harmonize all of our naming away from Extensions Gallery.