LearnDash LMS Notifications Addon 1.6.4

* Deprecate - Classes: `LearnDash\Notifications\Container`, `LearnDash\Notifications\App`.
* Tweak - Added compatibility with LearnDash Core v4.13.0.
* Tweak - Updated functions: `learndash_notifications_extra_autoloading`.

Feature: Add "Exclude pre-ordered course" option on Enroll into a course trigger.
Tweak: Better object select2 fields (course, lesson, etc) search results to support pagination.
Fix: Undefined array key error on Essay question has been put into graded status trigger.
Fix: Any object option in object field (course, lesson, etc) not saving.
Fix: Fatal error when using Assignment has been uploaded trigger and Assignment has been uploaded condition.
Fix: A scheduled lesson is available to user email not sending.

Feature: new action hooks 'learndash_notifications_email_sent' and 'learndash_notifications_email_failed'.
Fix: triggering objects settings in notification model don't work correctly filtering events that triggers set up notifications.
Fix: user can't select specific quiz associated with a lesson because quiz was only direct child to topic. Now it can be direct child of course, lesson, or topic.