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.

config/schedule.rb

@@ -22,7 +22,7 @@ if @cron_group == 'registry'
   end
 
   every :day, at: '12:10am' do
-    runner 'Invoice.cancel_overdue_invoices'
+    rake 'invoices:cancel_overdue'
   end
 
   # TODO