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
Oh, I guess that would only work if you used the `class_name` argument, and matched it to the name you assign it to.

Still, I think the error message might be more clear and accurate if it said "Managers created dynamically using `from_queryset` must be importable: inherit from them or assign them to a module-level name." or similar.