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
`Manager` does have a `__eq__`: https://github.com/django/django/pull/3687/files#diff-53c7979f86107903c879e78e12791851R233

But I agree, it should be `'objects'`. This also means, I'm missing a test case ;)