Get rid of national Inapplicability deprecation warning

The massaging of model attributes to get the `_delete` double-negative checkbox to behave as required for nation inapplicability nested attributes was raising a deprecation warning. I've replaced nested attributes with a custom writer that moves a chunk of the logic out of the controller and into the model. This isn't a perfect solution, but it gets rid of the deprecation warning and makes the code easier to test.