Core Data Manager should listen for Terminate

[regnerjr]

Right now core data does its final save, when the app terminates by inserting itself into

    func applicationWillTerminate(_ application: UIApplication) {
        CoreDataManager.sharedInstance.saveContext()
    }

It would be really great if the CoreDataManager could just listen for the NotificationCenter notification for UIApplicationWillTerminate documented here.

https://developer.apple.com/reference/foundation/nsnotification.name/1623061-uiapplicationwillterminate

This will help our app delegate be smaller and isolate our Core Data Manager from other parts of our app.