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

Ticket: https://code.djangoproject.com/ticket/23822

- [x] Implementation
- [x] User documentation
- [ ] Reference documentation
- [x] Tests
I don't see a way where a `CreateModel`/`DeleteModel` operation can occur in the same migration as a `AlterModelManagers` operation. Either the model isn't there in the beginning then it is created with all its managers, or it's there in the beginning but is gone in the end. The `AlterModelManagers` operation can only occur somewhere between two migration states.