* Changes to the paginator defaults and settings

Require a default paginator be specified when using the page size
setting.
https://github.com/encode/django-rest-framework/issues/5168

* DRF-5168 import warnings

missed this in last commit

* Add a system checks file

Add a check for pagination settings for the 3.7 upgrade cycle.

* more compatible import approach

* missing bactic

* revised language and approach to import the system check

Adds a rest framework app config.

* Adjust doc wording