Refactor EditItemViewController with AddItemViewController

[garricn]

As is, I'm keeping tracking of whether the user is Editing an item or Adding an item with private var selectedIndexPath: IndexPath? which works but presents some issues:

1. Its private so I can't test is but it doesn't need to be exposed
2. It makes testing TableViewModel more difficult because its private

Proposed solution:

1. Create an AddItemViewController. Make things simple. Use AddVC, when adding and EditVC when editing.
2. Call two different TableViewModelingDelegate methods
3. Call two different TableViewModelConfigurable methods

Lets see what happens when we do this.