Fixed documentation of disable_constraint_checking

The docstring and base implementation of disable_constraint_checking do not indicate that a return value is expected for proper behavior.
Fixed #11160 - Ensure full_clean is called from non_form_errors

Updated FormSet.non_form_errors() to ensure full_clean() has
been called before returning the errors.
Added myself to AUTHORS
Added example of using sitemaps with static views.

References #16829.
Fixed #20142 -- Added error handling for fixture setup

TestCase._fixture_setup disables transactions so,
in case of an error, cleanup is needed to re-enable
transactions, otherwise following TransactionTestCase
would fail.
Fix NoneType error when fetching a stale ContentType with get_for_id

When a stale ContentType is fetched, the _add_to_cache() function
didn't detect that `model_class()` returns `None` (which it does by
design). However, the `app_label` + `model` fields can be used instead
to as local cache key. Third party apps can detect stale models by
checking whether `model_class()` returns `None`.

Ticket: https://code.djangoproject.com/ticket/20442
Fixed #14894 -- Ensure that activating a translation doesn't run into threading issues.

Thanks to maxbublis for the report and sergeykolosov for the patch.
Fixed #15201: Marked CACHE_MIDDLEWARE_ANONYMOUS_ONLY as deprecated
Merge branch 'master' of github.com:django/django
Merge pull request #1127 from filias/20142

Fixed #20142 -- Added error handling for fixture setup
Fixes a Python 3.x regression introduced in a19e9d80
Merge pull request #1135 from ambv/model_validation_fix

Fixes a Python 3.x regression introduced in a19e9d80
Fixed #20284 - Added a note about DEBUG in static file docs.

Thanks wim@ for the suggestion and Nicolas Noé for the patch.
Fixed #13958 -- problem reporting exception from \r-line-ended file

Thanks petrvanblokland for reporting and saz for the patch
Add missing imports and models to the examples in the the model layer documentation
Add missing imports and models to the examples in the view layer documentation
Add missing imports to the examples in the 'First Steps'
add Leandra Finger, Silvan Spross and Marc Egli to AUTHORS
Fixed ff881aef on Python 3.
Fixed #20234 and #20236 -- SingleObjectMixin fixes

Added object on SingleObjectMixin returned context,
some code clanup.