Too much field exclusions in form's construct_instance() in _post_clean()
could lead to some unexpected missing ForeignKey values.
Fixes a regression from 45e04993. Refs #13776.

The RegexURLResolver.resolve() method no longer returns a tuple.
It has returned a ResolverMatch object since commit e0fb90b2.

Thanks jwa and lamby for the suggestions, and timgraham and jarshwah
for their reviews.

Fixed #25296 -- Prevented model related object cache pollution when create() fails due to an unsaved object.

Moved data loss check when assigning to a reverse one-to-one relation on
an unsaved instance to Model.save(). This is exactly the same change as
e4b813c0 but for reverse relations.

urlparse() fails with an AttributeError ("'__proxy__' object has no
attribute 'decode'") if reverse_lazy is used to look up the URL
(this is exactly the same problem that caused ticket #18776). The
solution is to use force_str() on the path before handing it to
urlparse().

Thanks Tim Graham for the review.

The change partly goes back to the old behavior for forwards migrations
which should reduce the amount of memory consumption (#24745). However,
by the way the current state computation is done (there is no
`state_backwards` on a migration class) this change cannot be applied to
backwards migrations. Hence rolling back migrations still requires the
precomputation and storage of the intermediate migration states.

This improvement also implies that Django does not handle mixed
migration plans anymore. Mixed plans consist of a list of migrations
where some are being applied and others are being unapplied.

Thanks Andrew Godwin, Josh Smeaton and Tim Graham for the review as well
as everybody involved on the ticket that kept me looking into the issue.

The ``item_enclosures`` hook returns a list of ``Enclosure`` objects which is
then used by the feed builder. If the feed is a RSS feed, an exception is
raised as RSS feeds don't allow multiple enclosures per feed item.

The ``item_enclosures`` hook defaults to an empty list or, if the
``item_enclosure_url`` hook is defined, to a list with a single ``Enclosure``
built from the ``item_enclosure_url``, ``item_enclosure_length``, and
``item_enclosure_mime_type`` hooks.

This doesn't work in Django 1.8 but was fixed in Django 1.9 as a
side effect of another change.

The issue was fixed on master due to the removal of ValuesQuerySet.

Partial forwardport of 7d471fe6 from stable/1.8.x

These steps are no longer needed as of pysqlite 2.7.0.
https://github.com/ghaering/pysqlite/commit/76f34850be582249d9048e6acb2993943b07d31b

Thanks Seth Gottlieb for help with the documentation and
Carl Meyer and Joshua Kehn for reviews.