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.

tests/many_to_one/tests.py

@@ -4,7 +4,7 @@ from copy import deepcopy
 from django.core.exceptions import FieldError, MultipleObjectsReturned
 from django.db import models, transaction
 from django.db.utils import IntegrityError
-from django.test import TestCase
+from django.test import TestCase, ignore_warnings
 from django.utils import six
 from django.utils.deprecation import RemovedInDjango20Warning
 from django.utils.translation import ugettext_lazy
@@ -580,6 +580,7 @@ class ManyToOneTests(TestCase):
         with self.assertNumQueries(1):
             self.assertEqual(th.child_set.count(), 0)
 
+    @ignore_warnings(category=RemovedInDjango20Warning)  # for use_for_related_fields deprecation
     def test_related_object(self):
         public_school = School.objects.create(is_public=True)
         public_student = Student.objects.create(school=public_school)
@@ -608,6 +609,16 @@ class ManyToOneTests(TestCase):
         finally:
             School._default_manager.use_for_related_fields = False
 
+        School._meta.base_manager_name = 'objects'
+        School._meta._expire_cache()
+        try:
+            private_student = Student.objects.get(pk=private_student.pk)
+            with self.assertRaises(School.DoesNotExist):
+                private_student.school
+        finally:
+            School._meta.base_manager_name = None
+            School._meta._expire_cache()
+
     def test_hasattr_related_object(self):
         # The exception raised on attribute access when a related object
         # doesn't exist should be an instance of a subclass of `AttributeError`