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
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 #18990: Loaddata now complains if fixture doesn't exist

The fixture named "initial_data" is exceptional though; if it
doesn't exist, the error is not raised. This allows syncdb and
flush management commands to attempt to load it without causing
an error if it doesn't exist.
Fixed #18990 -- Loaddata now complains if fixture doesn't exist

If the fixture doesn't exist, loaddata will output a warning.

The fixture named "initial_data" is exceptional though; if it
doesn't exist, the warning is not emitted. This allows syncdb and
flush management commands to attempt to load it without causing
spurious warnings.

Thanks to Derega, ptone, dirigeant and d1ffuz0r for contributions
to the ticket.
Fixed #12674 -- provide a way to override admin validation

Moved admin validation code to classes and have those be class
attributes to the ModelAdmin classes.
fix warnings imports in fixtures tests
Fixed #11915: generic Accept-Language matches country-specific variants
Fixed some line wrapping, refs #20233
Merge pull request #1134 from senko/ticket_18990

Fixed #18990: Loaddata now complains if fixture doesn't exist
Merge pull request #1094 from senko/ticket_11160

Fixed #11160: Formset non_form_errors returns ErrorList() if is_valid is not called
Merge pull request #1152 from ambv/issue11915

Fixed #11915: generic Accept-Language matches country-specific variants
Fixed #12747 -- Made reason phrases customizable.
Merge pull request #1028 from manfre/patch-2

Fixed #20340 - document required return value for disable_constraint_checking
Fixed #20126 -- XViewMiddleware moved to django.contrib.admindocs.middleware
Merge pull request #1138 from ambv/issue20126

Fixed #20126 -- XViewMiddleware moved to django.contrib.admindocs.middleware
Fixed #11442 -- Postgresql backend casts all inet types to text
Add missing imports and models to the examples in the template layer documentation
Add missing imports and models to the examples in security documentation