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/integration/tasks/data_migrations/regenerate_registrar_reference_numbers_test.rb

@@ -35,7 +35,7 @@ class RegenerateRegistrarReferenceNumbersTaskTest < ActiveSupport::TestCase
   def test_keeps_iso_reference_number_on_the_invoice_unchanged
     registrar = registrars(:bestnames)
     registrar.update_column(:reference_no, 'RF1111')
-    invoice = registrar.invoices.first
+    invoice = invoices(:one)
     invoice.update!(reference_no: 'RF2222')
 
     capture_io { run_task }