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
Not sure why it isn't documented, but I think it exists for this use case -- so you can control the name of the created class.

If you don't want to mention the assign-to-name option, that's fine. I would still suggest replacing the sentence beginning "Please note that" with this:

> Managers created dynamically using `Manager.from_queryset(...)` are not importable; subclass them and use the subclass.