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/custom_managers/tests.py

@@ -6,8 +6,9 @@ from django.utils import six
 
 from .models import (
     Book, Car, CustomManager, CustomQuerySet, DeconstructibleCustomManager,
-    FunPerson, OneToOneRestrictedModel, Person, PersonFromAbstract,
-    PersonManager, PublishedBookManager, RelatedModel, RestrictedModel,
+    FastCarAsBase, FastCarAsDefault, FunPerson, OneToOneRestrictedModel,
+    Person, PersonFromAbstract, PersonManager, PublishedBookManager,
+    RelatedModel, RestrictedModel,
 )
 
 
@@ -558,6 +559,34 @@ class TestCars(TestCase):
             ],
             lambda c: c.name
         )
+        # explicit default manager
+        self.assertQuerysetEqual(
+            FastCarAsDefault.cars.order_by("name"), [
+                "Corvette",
+                "Neon",
+            ],
+            lambda c: c.name
+        )
+        self.assertQuerysetEqual(
+            FastCarAsDefault._default_manager.all(), [
+                "Corvette",
+            ],
+            lambda c: c.name
+        )
+        # explicit base manager
+        self.assertQuerysetEqual(
+            FastCarAsBase.cars.order_by("name"), [
+                "Corvette",
+                "Neon",
+            ],
+            lambda c: c.name
+        )
+        self.assertQuerysetEqual(
+            FastCarAsBase._base_manager.all(), [
+                "Corvette",
+            ],
+            lambda c: c.name
+        )
 
 
 class CustomManagersRegressTestCase(TestCase):