Standardize extensions marketplace terminology

MR:

• chore: Rename some ExtensionsGallery to Extensi... (gitlab-web-ide!424 - merged)
• chore: Rename `WebIdeConfig.extensionsGallerySe... (gitlab-web-ide!425 - merged)
• chore: Finish renaming "gallery" to "marketplace" (gitlab-web-ide!426 - merged)
• Update extensions_gallery internal setting name... (!180981 - merged)
• Update ExtensionsGallery files to ExtensionsMar... (!180988 - merged)
• Rename extensionsGallery names to extensionMark... (!181010 - merged)

Description

At present, we use two different terms to refer to the extensions marketplace in the codebase: "extensions marketplace" and "extensions gallery". Let's settle on one term and adopt it as the standard moving forward.

From #508991 (comment 2264837474)+

Acceptance Criteria

• Use "Extension Marketplace" instead of "Extensions Gallery" in the code when possible.