Refactor and improve invoices

- `runner 'Invoice.cancel_overdue_invoices'` in `schedule.rb` is
changed to `rake 'invoices:cancel_overdue'`.
- `invoices.payment_term` database column is removed and its value is
hardcoded in UI.
- `invoices.paid_at` is removed as unused
- `invoices.due_date` column's type is now `date`.
- `Invoice#invoice_items` renamed to `Invoice#items` and `Invoice`
interface to get a list of items is unified.
- Default date format in UI.
- Default translations are used.
- Tests improved.
- Specs converted to tests and removed along with factories.
- Database structure improved.

test/models/registrar_test.rb

@@ -83,4 +83,17 @@ class RegistrarTest < ActiveSupport::TestCase
       registrars(:goodnames).update!(reference_no: '1234')
     end
   end
+
+  def test_issues_new_invoice
+    travel_to Time.zone.parse('2010-07-05')
+    @original_days_to_keep_invoices_active_setting = Setting.days_to_keep_invoices_active
+    Setting.days_to_keep_invoices_active = 10
+
+    invoice = @registrar.issue_prepayment_invoice(100)
+
+    assert_equal Date.parse('2010-07-05'), invoice.issue_date
+    assert_equal Date.parse('2010-07-15'), invoice.due_date
+
+    Setting.days_to_keep_invoices_active = @original_days_to_keep_invoices_active_setting
+  end
 end