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
The construction "As for..." often means "Regarding" - I think "As with custom fields" would be clearer here. Is there more extensive documentation about the back-compat situation with custom fields that we could link to from here?