Replaced the custom, untested memoize with a similar decorator from Python's
3.2 stdlib. Although some minor performance degradation (see ticket), it is
expected that in the long run lru_cache will outperform memoize once it is
implemented in C.

Thanks to EvilDMP for the report and Baptiste Mispelon for the idea of
replacing memoize with lru_cache.

Corrected LANGUAGES documentation on how to translate language
names. Now using django.utils.translation.ugettext_lazy instead
of a dummy gettext() function.

Thanks to Salvatore for the report.

Filtering out static file requests in runserver has been judged
arbitrary and can hide some debugging-related activity.
Thanks Roy Smith for the report and Aymeric Augustin for the
review.

Thanks arjan at anymore.nl for the report.

Refs #16969.

Thanks Andreas Pelme for the report and initial patch, and
Aymeric Augustin, Shai Berger and Tim Graham for the reviews.

Fixed #21172 -- have LiveServerThread follow the semantics of threading.Thread.join()

The EmailMessage documentation describes the initialization parameters
and states they 'can be set at any time prior to calling the send()
method.'. However, the 'headers' parameter's corresponding attribute is
called 'extra_headers'.

Thanks Reza Mohammadi for the report.

``get_template_from_string`` default arguments were breaking
``assertTemplateUsed``. The solution has been to return only the names of the
templates with a ``name`` attribute distinct of ``None``. The default ``name``
kwarg of ``Template`` has been changed to ``None``, more pythonic than ``'<Unknown
Template>'``.

Thanks mitar for the report and Tim Graham for the review.

Refs #14461. Thanks Ramiro Morales for pointing this.

Thanks to Tim for the review.

Add the trimmed option to the blocktrans tag to trim any newlines and
whitespace from its content.

This allows the developer to indent the blocktrans tag without adding
new lines and whitespace to the msgid in the PO file.

Thanks to mpessas for the initial patch and Dmitri Fedortchenko for the
report.

Fixed #13725 -- take url scheme into account in assertRedirects

Thanks to Loic for review.

Scheme is handled correctly when making comparisons between two URLs. If
there isn't any scheme specified in the location where we are redirected to,
the original request's scheme is used. If present, the scheme in
``expected_url`` is the one used to make the comparations to.

 * Safer for use in multiprocess environments
 * Better random culling
 * Cache files use less disk space
 * Safer delete behavior

Also fixed #15806, fixed #15825.

This patch introduces the Prefetch object which allows customizing prefetch
operations.

This enables things like filtering prefetched relations, calling select_related
from a prefetched relation, or prefetching the same relation multiple times
with different querysets.

When a Prefetch instance specifies a to_attr argument, the result is stored
in a list rather than a QuerySet. This has the fortunate consequence of being
significantly faster. The preformance improvement is due to the fact that we
save the costly creation of a QuerySet instance.

Thanks @akaariai for the original patch and @bmispelon and @timgraham
for the reviews.

Instead always create new joins as OUTER.

This commit introduced a new class JoinPromoter that can be used to
abstract away join promotion problems for complex filter conditions.
Query._add_q() and Query.combine() now use the new class.

Also, added a lot of comments about why join promotion is done the way
it is.

Thanks to Tim Graham for original report and testing the changes, and
for Loic Bistuer for review.