#20432: Fix for GroupAdmin test
Fixed #9595 -- Allow non-expiring cache timeouts.

Also, streamline the use of 0 and None between cache backends.
Merge pull request #1088 from jacobb/upstream

#20432: Fix for GroupAdmin test
Fixed #18998 - Prevented session crash when auth backend removed

Removing a backend configured in AUTHENTICATION_BACKENDS should not
raise an exception for existing sessions, but should make already
logged-in users disconnect.
Thanks Bradley Ayers for the report.
Slightly reworked imports in contrib.auth.__init__
Added stripping of whitespace for SlugField and URLField
Merge pull request #1113 from denibertovic/master

Fixed #18761 -- Added whitespace stripping to URLField and SlugField.
Fixed #19436 -- Don't log warnings in ensure_csrf_cookie.
Moved IgnorePendingDeprecationWarningsMixin in django.test.utils.

This mixin is useful whenever deprecating a large part of Django.
Fixed #20444 -- Cookie-based sessions does not include a remote code execution-warning