Make country_select to use new LegacyCountry model

This quacks like the new WorldLocation model, and therefore will allow
switching datasets on request.
Update flows to use LegacyCountry model

Also remove some redundant methods
Add :use_legacy_data option to country_select

Update all existing flows etc to use this option.  Not using it currently raises an exception until the new data has been hooked up.
Merge pull request #324 from alphagov/legacy_countries

Add :use_legacy_data option to country_select