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.

docs/ref/models/options.txt

@@ -35,6 +35,16 @@ Available ``Meta`` options
     or ``app_label.model_name`` you can use ``model._meta.label``
     or ``model._meta.label_lower`` respectively.
 
+``base_manager_name``
+---------------------
+
+.. attribute:: Options.base_manager_name
+
+.. versionadded:: 1.10
+
+The name of the manager to use for the model's
+:attr:`~django.db.models.Model._base_manager`.
+
 ``db_table``
 ------------
 
@@ -95,6 +105,16 @@ Django quotes column and table names behind the scenes.
     setting, if set. If the backend doesn't support tablespaces, this option is
     ignored.
 
+``default_manager_name``
+------------------------
+
+.. attribute:: Options.default_manager_name
+
+.. versionadded:: 1.10
+
+The name of the manager to use for the model's
+:attr:`~django.db.models.Model._default_manager`.
+
 ``default_related_name``
 ------------------------