Thanks Tim Graham for polishing the patch, updating the tests, and
writing documentation. Thanks Carl Meyer for shepherding the DEP.

Fixed #10506, #13793, #14891, #25201 -- Introduced new APIs to specify models' default and base managers.

This deprecates use_for_related_fields.

Old API:

class CustomManager(models.Model):
    use_for_related_fields = True

class Model(models.Model):
    custom_manager = CustomManager()

New API:

class Model(models.Model):
    custom_manager = CustomManager()

    class Meta:
        base_manager_name = 'custom_manager'

Refs #20932, #25897.

Thanks Carl Meyer for the guidance throughout this work.
Thanks Tim Graham for writing the docs.

Thanks Tim Graham for the review.

Thanks Daniel Wiesmann for the report and the appropriate fixes.

Fields inherited from abstract base classes may be overridden like
any other Python attribute. Inheriting from multiple models/classes
with the same attribute name will follow the MRO.

Thanks Markus and Tim for the review.

Thanks Markus for your contribution and Tim for your review.

This code added in 107c9f54 isn't
used after 4ce7a6bc.

The issue was fixed by 8a47ba67
(refs #16508).

Thanks Tim for the review.

Thanks Tom Christie for review.