Fixed #18389 -- Fixed the way contribute_to_class is called

Now this method is only called only if the object is an instance.
This allows to have field classes as model class attributes.
Fixed #22622 -- Added ugettext_lazy import in docs
Fixed #22167 -- Improved documentation on context processors

Clarified the explanation on the order in which user provided variables
to render_to_response, RequestContext and context processors are
loaded.
Fixed #22266 - quote PK before redirecting away from add_view (django.contrib.admin)
Fixed #15716 - Authentication backends can short-circuit authorization.

Authorization backends can now raise PermissionDenied in "has_perm"
and "has_module_perms" to short-circuit authorization process.