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
Fixed #13285: populate_xheaders breaks caching
Merge pull request #1122 from ambv/issue13285

Fixed #13285: populate_xheaders breaks caching
Fixed #20430 - Enable iterable of iterables for model choices

Allows for any iterable, not just lists or tuples, to be used as
the inner item for a list of choices in a model.
Fixed #20446 -- Documentation for SmallIntegerField does not clarify 'small'
Fixed #20335 - Documented the {% language %} template tag.

Thanks bmispelon for the suggestion and djangsters for the patch.
Merge pull request #1128 from erikr/improve-smallint-docs

Fixed #20446 -- Documentation for SmallIntegerField does not clarify 'small'
Merge branch 'allow-any-iterable-for-choices'