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/admin/invoices_controller.rb

@@ -33,13 +33,8 @@ module Admin
     end
 
     def cancel
-      if @invoice.cancel
-        flash[:notice] = t(:record_updated)
-        redirect_to([:admin, @invoice])
-      else
-        flash.now[:alert] = t(:failed_to_update_record)
-        render :show
-      end
+      @invoice.cancel
+      redirect_to [:admin, @invoice], notice: t('.cancelled')
     end
 
     def forward