Dropped the DJANGO_TEST_TEMP_DIR environment variable.

Before this change, proper removal depended on the developer passing
dir=os.environ['DJANGO_TEST_TMP_DIR'] to tempfile functions.

The website only renders code blocks at 96 chars, and therefore
long code lines get wrapped. Manually breaking the lines prevents
the wrapping from occurring.

These cached properies were causing problems with pickling, and in
addition they were confusingly defined: field.rel.model._meta was
not the same as field.rel.opts.

Instead users should use field.rel.related_model._meta inplace of
field.rel.opts, and field.rel.to._meta in place of field.rel.to_opts.

Switched from an adjancency list and uncached, iterative depth-first
search to a Node-based design with direct parent/child links and a
cached, recursive depth-first search. With this change, calculating
a migration plan for a large graph takes several seconds instead of
several hours.

Marked test `migrations.test_graph.GraphTests.test_dfs` as an expected
failure due to reaching the maximum recursion depth.

Refs #24389.

The stated reason for its introduction in d18d37ce no longer applies
since Django's code repository was switched from Subversion to git.

Furthermore it never had any effect because shutil.rmtree ignores its
onerror argument when ignore_errors is True.

The reason for its use in template management commands is unclear.

Thanks uranusjr for the report and analysis.

In addition to simplifying the code, this reduces the number of writes.

This makes the staticfiles tests 2.5 times faster.

It should have been removed when the ADMIN_MEDIA_PREFIX setting was deprecated.

Refs #24351.

Explicit is better than implicit.

The new signature enables better support for routing RunPython and
RunSQL operations, especially w.r.t. reusable and third-party apps.

This commit also takes advantage of the deprecation cycle for the old
signature to remove the backward incompatibility introduced in #22583;
RunPython and RunSQL won't call allow_migrate() when when the router
has the old signature.

Thanks Aymeric Augustin and Tim Graham for helping shape up the patch.

Refs 22583.

Thanks to DavidMuller for the report.

As suggested by Anssi. This has the slightly strange side effect of
passing the expression to Expression.convert_value has the expression
passed back to it, but it allows more complex patterns of expressions.

Joint effort between myself, Josh, Anssi and Shai.

This opens more possibilities, like accessing context.template.origin.

It also follows the chain of objects instead of following a shortcut.

Workaround for http://bugs.python.org/issue20747.
In some corner cases, Python 2 inserts a newline in a header value
despite `maxlinelen` passed in Header constructor.
Thanks Tim Graham for the review.

Thanks Joris Benschop for the report, and Tim Graham for the tests.