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/internals/deprecation.txt

@@ -175,6 +175,12 @@ details on these changes.
 * The ``escape`` filter will change to use
   ``django.utils.html.conditional_escape()``.
 
+* ``Manager.use_for_related_fields`` will be removed.
+
+* Model ``Manager`` inheritance will follow MRO inheritance rules and the
+  ``Meta.manager_inheritance_from_future`` to opt-in to this behavior will be
+  removed.
+
 .. _deprecation-removed-in-1.10:
 
 1.10