Add option to save changes for later

[CSchmitz81]

Users may want to save their changes for later, or if their session ends unexpectedly they may want to resume editing where they left off. We should persist editing changes in a user's local history so they can resume them (from the notification bar on the right). This can include segment variations, description config changes, and new elements without a required parent. There should also be an option to save changes for later manually, which would behave like cancel except cancel should remove the changes from the user's local history (i.e. users can't resume canceled changes).