Clearer explanation when exception has no message

"No exception supplied" is misleading; actually there is an exception,
but there's no message string.
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.
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
Make Urdu (ur) recognized as RTL language. Refs #20454
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.
Merge pull request #1142 from Pike/master

Fixed #20454: Make Urdu (ur) recognized as RTL language
Improve cookie based session backend docs.

Note the don't require sessions to be in installed apps.
Fix a typo in a comment.
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 #20452 -- Rename 'headers' to 'header fields'.
Added TransRealMixin to fix i18n global state pollution in the test suite
Merge pull request #1147 from ambv/transrealmixin

Added TransRealMixin to fix i18n global state pollution in the test suite
Fixed a rest mistake I introduced in d5ce2ff.
Merge pull request #1098 from zsiciarz/ticket-16829

Added example of using sitemaps with static views.
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