FormKit support for Notification Settings

![](http://i.imgur.com/ahI8egI.gif)
They are not. We did that before this PR, and it doesn't work, because some `NSIndexPath` values that we're passed are secretly `UIMutableIndexPath` (a private UIKit subclass of `NSIndexPath`). The `UIMutableIndexPath`s don't act as the same key as their corresponding `NSIndexPath`, which is strange and sad.