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.

app/controllers/registrar/payments_controller.rb

@@ -29,7 +29,7 @@ class Registrar
       if @payment.valid_response_from_intermediary? && @payment.settled_payment?
         @payment.complete_transaction
 
-        if invoice.binded?
+        if invoice.paid?
           flash[:notice] = t(:pending_applied)
         else
           flash[:alert] = t(:something_wrong)