merge automatic group order invoice generation

see https://github.com/foodcoops/foodsoft/pull/907 for reference
and original work by viehlieb

Co-authored-by: viehlieb <pf@pragma-shift.net>

fix PDF Pdf

db/migrate/20211208142719_create_group_order_invoices.rb

@@ -0,0 +1,13 @@
+class CreateGroupOrderInvoices < ActiveRecord::Migration[5.2]
+  def change
+    create_table :group_order_invoices do |t|
+      t.integer :group_order_id
+      t.bigint :invoice_number, unique: true, limit: 8
+      t.date :invoice_date
+      t.string :payment_method
+
+      t.timestamps
+    end
+    add_index :group_order_invoices, :group_order_id, unique: true
+  end
+end