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/models.py

@@ -172,6 +172,18 @@ class Car(models.Model):
         return self.name
 
 
+class FastCarAsBase(Car):
+    class Meta:
+        proxy = True
+        base_manager_name = 'fast_cars'
+
+
+class FastCarAsDefault(Car):
+    class Meta:
+        proxy = True
+        default_manager_name = 'fast_cars'
+
+
 class RestrictedManager(models.Manager):
     def get_queryset(self):
         return super(RestrictedManager, self).get_queryset().filter(is_public=True)