Change Details

Part of [ENG-4319](https://linear.app/comm/issue/ENG-4319/warn-the-user-when-deleting-a-chat-that-has-contained-chats). A new selector is needed to obtain contained chats.

Part of [ENG-4319](https://linear.app/comm/issue/ENG-4319/warn-the-user-when-deleting-a-chat-that-has-contained-chats). A new selector is needed to obtain contained chats. There exists `childThreadInfos` selector, but it operates on `parentThreadID` rather than on `containingThreadID`.

Part of [ENG-4319](https://linear.app/comm/issue/ENG-4319/warn-the-user-when-deleting-a-chat-that-has-contained-chats). A new selector is needed to obtain contained chats. There exists `childThreadInfos` selector, but it operates on `parentThreadID` rather than on `containingThreadID`.