Shipments download

Adds a raw CSV download of shipments ("shipment by shipment") for the admin interface. The report is generated using the COPY command and for now has no size constraint.

To simplify the generation of the report and to provide consistency for future reports, the search is now based on a  view. Thir required small changes to the Trade::Filter to ensure we're arriving at the same results through the web search and CSV download.

You need to run `rake db:migrate`