Fire an action when order limits are active

When `OrderLimiter::disable_ordering()` is called, an action should be fired to let other plugins get in on the action.

```php
/**
 * Fires when ordering has been disabled.
 *
 * @param \Nexcess\LimitOrders\OrderLimiter $limiter The order limiter instance.
 */
do_action( 'limit_orders_disable_ordering', $this );
```