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
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 #20234 and #20236 -- SingleObjectMixin fixes

Added object on SingleObjectMixin returned context,
some code clanup.
Typo in comment
Make Urdu (ur) recognized as RTL language. Refs #20454
Merge pull request #1116 from elektrrrus/ticket_20234_20236

Ticket 20234 20236
add comment to explain why it's necessary to do an explicit check for
Python2 and how metaclasses are defined
Merge pull request #1129 from frog32/master

Add needed Imports to the Documentation
Merge pull request #1141 from inglesp/typo

Typo in comment
refs #20233 - Full custom user model example isn't really full

Addition and fix for custom user model example documentation.

https://code.djangoproject.com/ticket/20233
Merge pull request #1143 from AndreaCrotti/master

A simple docstring to clarify a part of the code that might be confusing to understand
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 #1145 from postrational/ticket_20233

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