Whole-site imports record the stock they lower

A whole-site import now records the stock it lowers. When a file imported with "Add missing" lowers a model's stock in the Prices & stock table of a product that sells by catalogue, the decrease is recorded in Products › Inventory › Write-offs as «Correction», from «Import › Full site», with who ran the import. If the units turn up, undo the record - they go back to the same row.