Add a button to trigger GnuCash CSV generation to the invoice_form view

This button is only there if information has been filled in for the
company and customer.

stoptime.rb

@@ -1388,6 +1388,8 @@ module StopTime::Controllers
       tex_file = PUBLIC_DIR + "invoices/#{@number}.tex"
       pdf_file = PUBLIC_DIR + "invoices/#{@number}.pdf"
       csv_file = PUBLIC_DIR + "invoices/#{@number}.csv"
+      @csv_enabled = @company.gnucash_revenues_account_name.present? &&
+                     @customer.gnucash_customer_owner_id.present?
       if @format == "html"
         @input = @invoice.attributes
         @invoice_file_present = tex_file.exist? || csv_file.exist?
@@ -2652,6 +2654,14 @@ module StopTime::Views
             _icon("download")
             span "Download LaTeX"
           end
+          if @csv_enabled
+            a.btn.btn_default role: "button",
+              href: R(CustomersNInvoicesX,
+                      @customer.id, "#{@invoice.number}.csv") do
+              _icon("download")
+              span "Download GnuCash CSV"
+            end
+          end
           a.btn.btn_default role: "button",
             href: R(Company, revision: @company.revision) do
             _icon("briefcase")