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
Also, I'm not sure about this message. Do you actually have to inherit, or is it enough to have it as a module-level name, e.g. `MyManager = models.Manager.from_queryset(MyQuerySet)` at module level?