Changed push to modal segue

- Went back to Add To-Do Item VC being modally presented by the
    To-Do List VC.
- For this to work, I needed to store the new item in the To-Do List
    VC rather than the Add To-Do Item VC.
Fixed bug where items could not be moved upwards

- Needed to get the index of the item to be moved sooner, before
    updating the indices of the other items