reverse of OneToOne relation should use default manager if use_for_related_fields specified

[Shaun Cutts]

When retrieving the related object in a reverse OneToOne relation, the SingleRelatedObjectDescriptor doesn't respect "use_for_related_fields" if defined by the default manager for the class of the related object. See patch for proposed fix (which indicates the code in question as well).

[Shaun Cutts]

patch for SingleRelatedObjectDescriptor

[Aymeric Augustin]

Change UI/UX from NULL to False.

[Aymeric Augustin]

Change Easy pickings from NULL to False.

[Loic Bistuer]

​PR6175 deprecates use_for_related_fields. "to-one" relations go through _base_manager which can be specified with the base_manager_name model option.

[Loïc Bistuer <loic.bistuer@…>]

In ed0ff91:

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.