Add missing imports and models to the examples in internationalization and localization documentation

Fixed #20340 - document required return value for disable_constraint_checking

Fixed #11915: generic Accept-Language matches country-specific variants

Fixed #11160: Formset non_form_errors returns ErrorList() if is_valid is not called

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

Moved admin validation code to classes and have those be class
attributes to the ModelAdmin classes.

Added example of using sitemaps with static views.

Added TransRealMixin to fix i18n global state pollution in the test suite

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.

Note the don't require sessions to be in installed apps.

Fixed #20454: Make Urdu (ur) recognized as RTL language

Fixed #20233 - Full custom user model example isn't really full

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.

A simple docstring to clarify a part of the code that might be confusing to understand

Addition and fix for custom user model example documentation.

https://code.djangoproject.com/ticket/20233

Typo in comment

Add needed Imports to the Documentation

Python2 and how metaclasses are defined

Ticket 20234 20236

Thanks zalew for the suggestion and work on a patch.

Also updated, tweaked and fixed testing documentation.

Added object on SingleObjectMixin returned context,
some code clanup.