[Quiz] Deleting a quiz questions causes issues when grading / reviewing quiz attempts

### Reproduction Steps

+ Create and publish a quiz
+ Take the quiz as a student to create a quiz attempt (you can do this as the course creator/instructor)
+ Delete one or more questions from the published quiz
+ Review the quiz attempt as an admin & attempt to grade it or leave remarks

### Expected Behavior

+ The quiz should display information about the deleted question despite it being deleted

### Actual Behavior

+ The deleted question is not visibly displayed on the attempt grade
+ An error is encountered when saving the grade (see below)
+ The quiz still counts the deleted question for points, number of questions, and grading purposes
+ The widgets on the quiz attempt page display this quiz in counts, grades, points, etc...

### Error Messages / Logs

+ Include any relevant error messages or log files
```
**[26-Apr-2019 18:16:39 UTC] PHP Fatal error: Uncaught Error: Call to a member function get_auto_grade_type() on null in /srv/www/llms-72/public_html/wp-content/plugins/lifterlms/includes/models/model.llms.quiz.attempt.question.php:250
Stack trace:
#0 /srv/www/llms-72/public_html/wp-content/plugins/lifterlms/includes/models/model.llms.quiz.attempt.php(553): LLMS_Quiz_Attempt_Question->get_status()
#1 /srv/www/llms-72/public_html/wp-content/plugins/lifterlms/includes/models/model.llms.quiz.attempt.php(122): LLMS_Quiz_Attempt->is_auto_gradeable()
#2 /srv/www/llms-72/public_html/wp-content/plugins/lifterlms/includes/controllers/class.llms.controller.admin.quiz.attempts.php(103): LLMS_Quiz_Attempt->calculate_grade()
#3 /srv/www/llms-72/public_html/wp-content/plugins/lifterlms/includes/controllers/class.llms.controller.admin.quiz.attempts.php(61): LLMS_Controller_Admin_Quiz_Attempts->save_grade(Object(LLMS_Quiz_Attempt))
#4 /srv/www/llms-72/public_html/wp-includes/class-wp-hook.php(286): LLMS_Controller_Admin_Quiz_Attempts->maybe_run_actions('')
#5 /srv/www/l in /srv/www/llms-72/public_html/wp-content/plugins/lifterlms/includes/models/model.llms.quiz.attempt.question.php on line 250
**

```

### Possible Solutions

+ When a question is deleted from a quiz LifterLMS should determine if the question exists in any existing attempts. If it does, the question should be *archived* instead of deleted.
+ This could be a new custom post status reserved for this post type.
+ An archived question should be excluded from all quiz related queries EXCEPT with relation to quiz attempts
+ When an attempt is deleted all archived questions should be checked to see if that archived question can now safely be deleted
+ An interface on the admin review may need to display that the question is an archived question (?)