Add UI for Group settings enterprise_users_extensions_marketplace_opt_in

MR: Add UI for enterprise user Extension Marketplac... (!161819 - merged)

Description

Once we have the Db field for this (see this issue), we'll want to create a UI for group admins to set this.

Acceptance Criteria

• UI added for controlling field enterprise_users_extensions_marketplace_opt_in.
• Permissions and licensed feature concerns taken into consideration.
• EE split concerns taken into consideration.