Add selectable content blocking targets

[wizdom13]

• Add an Apply blocking to multi-select setting with backward-compatible defaults
• Scope content blocking independently for search, kiosks, related items, channel pages, subscriptions, remote playlists, and comments
• Keep local history and local playlists outside remote content blocking
• Refresh active lists when target selections change
• Add behavioral coverage for default, selected, and empty target sets
• Fixes Specify content blocking targets #497