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/proxy_models/models.py

@@ -85,6 +85,8 @@ class StatusPerson(MyPerson):
     """
     status = models.CharField(max_length=80)
 
+    objects = models.Manager()
+
 # We can even have proxies of proxies (and subclass of those).
 
 
@@ -96,6 +98,8 @@ class MyPersonProxy(MyPerson):
 class LowerStatusPerson(MyPersonProxy):
     status = models.CharField(max_length=80)
 
+    objects = models.Manager()
+
 
 @python_2_unicode_compatible
 class User(models.Model):