Fix gallery notifications timestamps

[mdashlw]

Before you begin

• I understand my contributions may be rejected for any reason
• I understand my contributions are for the benefit of Derpibooru and/or the Philomena software
• I understand my contributions are licensed under the GNU AGPLv3

• I understand all of the above

---

Galleries currently have only one instance of Notification where they are the actor and child actor is nil. Not having a child actor causes that notification to have the same created_at and updated_at fields because child actor is the only thing that ever changes in a Notification. The issue is that notifications page uses notifcation.updated_at timestamp, so currently it displays when the first image was added to a gallery which could be years ago. This PR resolves it by making child actor the added image which forces updated_at to update.