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 it seems like it has `object` where it should be `objects` (plural). Maybe this comparison isn't needed here, but at the very least it seems like dead code that should be removed?