Update Web IDE settings to use admin settings for vscode_extensions_gallery values

MR: Use vscode_extension_marketplace settings in We... (!184067 - merged)

Description

We want to update existing references to extensions marketplace settings in the Web IDE module to use the extensions gallery values from the admin settings.

Acceptance Criteria

• Remove hardcoded Open VSX settings from default settings (current implementation).
• Update extensions gallery metadata generator and validator to read extensions gallery values from admin settings. EDIT: While implementing !184067 (merged), it looks like validator update isn't needed.
• Allow vscode_extensions_gallery to be nil everywhere (in some places this is expected to always exist). EDIT: While implementing !184067 (merged), it looks like this is not needed.

Please note: We want to protect the existing web_ide_extensions_marketplace behavior, so please peruse this thread #508991 (comment 2308669780)