Forward port of 05a781a4 from stable/1.8.x

This change:

* Makes the InclusionNode cache-safe by removing render-time side effects
  to its nodelist.
* Ensures the render_context stack is properly scoped and reset by updating
  the render call to use Template.render rather than Nodelist.render.

Rendering a Jinja template with self in the context threw an error.
While self is a reserved variable in Jinja, including self in the
context is not an error, so Django should respect that.

Thanks Claude Paroz and Tim Graham for the review

Calling Migration.mutate_state() now also allows to do in_place
mutations in case an intermediate state is thrown away later.

Thanks Anssi Kääriäinen for the idea, Ryan Hall for parts of the patch,
and Claude Paroz and Tim Graham for the review

SimpleCookie.__repr__() changed in
https://hg.python.org/cpython/rev/88e1151e8e02

Thanks Aymeric Augustin and Florian Apolloner for work on the patch.

Thanks Florian Apolloner for the review.

Thanks simon29 for report, and freakboy3742, floledermann,
jacob, claudep and collinanderson for discussing the task.

Thanks Tim Graham for the review.

Thanks djbug for the report and Aymeric Augustin and Carl Meyer for the
review.

Made MigrationGraph forwards_plan() and backwards_plan() fall back to an
iterative approach in case the recursive approach exceeds the recursion
depth limit.

Thanks to David Sanders for the report and test and Simon Charette for
the review.

Thanks Tim Graham for reporting the regression.

Added a django.template logger without a default handler. Added
logging if there is an exception while resolving variables in a
template.

Added support for multiple m2m fields with the same 'to' model
and with related_name set to '+'.

This introduces a force_append_slash argument for request.get_full_path()
which is used by RedirectFallbackMiddleware and CommonMiddleware when
handling redirects for settings.APPEND_SLASH.

`','`, `'1,,1'`, `',1'` etc. are no longer considered as valid
comma-separated integer lists.

This adds a new method, Apps.lazy_model_operation(), and a helper function,
lazy_related_operation(), which together supersede add_lazy_relation() and
make lazy model operations the responsibility of the App registry. This
system no longer uses the class_prepared signal.