Use country codes to standardize around ISO 3166

I can't get the tests to run because they seem to rely on `ActionView` so I haven't changed them to reflect these changes.

This pull request is mostly my attempt to start a conversation around using the ISO 3166 codes as the values for countries since that seems like a much better lookup mechanism when persisting country data.

I've also extracted the countries into a separate file with a line for each country. This should make diffs easier to read when country names/codes change.

Please let me know what you think.