Always require invoice VAT rate

Closes #1031

lib/tasks/data_migrations/populate_invoice_vat_rate.rake

@@ -0,0 +1,16 @@
+namespace :data_migrations do
+  task populate_invoice_vat_rate: :environment do
+    processed_invoice_count = 0
+
+    Invoice.transaction do
+      Invoice.where(vat_rate: nil).find_each do |invoice|
+        vat_rate = Invoice::VatRateCalculator.new(registrar: invoice.buyer).calculate
+        invoice.update_columns(vat_rate: vat_rate)
+
+        processed_invoice_count += 1
+      end
+    end
+
+    puts "Invoices processed: #{processed_invoice_count}"
+  end
+end