Made it possible to register and run callbacks after a database
transaction is committed with the `transaction.on_commit()` function.

This patch is heavily based on Carl Meyers django-transaction-hooks
<https://django-transaction-hooks.readthedocs.org/>. Thanks to
Aymeric Augustin, Carl Meyer, and Tim Graham for review and feedback.

This reverts commit bd691f45 (refs #24177).

If the only manager on the model is the default manager defined
by Django (`objects = models.Manager()`), this manager will not
be added to the model state. If it is custom, it needs to be
passed to the model state.

This is a security hardening fix to help prevent XSS (and incorrect HTML)
for the common use case of simple_tag.

Thanks to Tim Graham for the review.

When the pk was a relation field, qs.filter(pk__in=qs) didn't work.

In addition, fixed Restaurant.objects.filter(place=restaurant_instance),
where place is an OneToOneField and the primary key of Restaurant.

A big thank you to Josh for review and to Tim for review and cosmetic
edits.

Thanks to Beauhurst for commissioning the work on this ticket.

As GDAL_VERSION is conditionnaly imported, it cannot be referenced
in skipIf decorator.

Thanks to Simon Charette for the report.

This is an alternative patch to the unsuccessful b43b27bc.

This reverts commit b43b27bc.
This commit broke tests initialization on systems without GIS libraries.

Partial revert of 61d09e61. At __del__ time, it might be to late to bind
those functions.

Thanks to Tim Graham for the report and Shai Berger for the review.

Forwardport of ae1d663b
from stable/1.8.x plus more.

Thanks to Tim Graham for the review.

By providing a .coveragerc file with all default settings, users only
have to execute "coverage run ./runtests.py" without the need to specify
all the possible flags. The same applies to "coverage html" and
"coverage xml".

Thanks Andriy Sokolovskiy and Simon Charette for the review.

Implemented __str__() to return the string-representation of the
proxied object, not the proxy itself, if the lazy object didn't have
a string-like object in its resultclasses.