Fixed #23822 -- Added support for serializing model managers in migration

Ticket: https://code.djangoproject.com/ticket/23822

- [x] Implementation
- [x] User documentation
- [x] Reference documentation
- [x] Tests
There you go: https://github.com/django/django/pull/3687/files#diff-5210414c7a29d5d1f61588388b517599R202