prereq and dependency checks on post trash

use action `wp_trash_post` (https://core.trac.wordpress.org/browser/tags/4.5.3/src/wp-includes/post.php#L2656) to perform lookups before trashing

Also check into untrash actions

### course

- [ ] remove prerequisites from any other courses
- [ ] remove from membership auto-enrollment
- [ ] cancel enrollments?
- [ ] delete child sections
- [x] remove access plans (on deletion) (via #1518, #1522)

### sections

- [ ] unset lesson parent meta data

### lesson

- [x] remove prerequisites from any other lessons
- [ ] warnings on builder when lessons deleted (empty node on syllabus)

### quiz

- [x] remove from any lessons
- [x] remove quiz attempts

### questions

- [x] remove from any quizzes

### track

- [ ] remove prerequisite from any courses

### membership

- [ ] remove restriction settings from any post which supports membership restrictions
- [ ] remove from members-only access plans
- [ ] check and remove from site wide restriction setting
- [ ] cancel enrollments?
- [x] remove access plans (on deletion) (via #1518, #1522)

### ~user~

- ~postmeta cleanup~
- ~order anonymization~
- ~cancel recurring orders~

User cleanup moved to #940

### orders

- [x] related transactions

WIP: https://github.com/gocodebox/lifterlms/blob/master/includes/class.llms.post.relationships.php