Exporter / Importer /  Country of Origin

This adds 2 validations:

* Export country + Origin country
* Export country + Import country

Both operate on the sandbox shipments view.

In the rails console:
Trade::PovDistinctValuesValidationRule.create(
  :column_names => ['exporter', 'country_of_origin'],
  :run_order => 4,
  :is_primary => false
)
Trade::PovDistinctValuesValidationRule.create(
  :column_names => ['exporter', 'importer'],
  :run_order => 4,
  :is_primary => false
)