Fixed #15961 -- Modified ModelAdmin to allow for custom search methods.

This adds a get_search_results method that users can override to
provide custom search strategies.

Thanks to Daniele Procida for help with the docs.
Merge pull request #1166 from bbenko/ticket_15961

Fixed #15961 -- Added get_search_results to ModelAdmin
Merge pull request #1178 from bmispelon/master

Fix for test failure introduced by 980ae2a.
Fixed #20183 - Clarified docs assumption that certain objects exist in database.

Thanks Tomasz Jaskowski for the patch.
Made 2d309a70 compatible with Python 3.
Added some links in /docs/intro/overview.txt

Thanks Claes Ström for the patch.
Fixed #16330 -- added --pks option in dumpdata command

Thanks to guettli for the initial ticket and patch, with additional work
from mehmetakyuz and Kevin Brolly.
Fixed #20459 - Improved example for setting HTTP header fields.

Thanks Jérémie Blaser.
Changed API to disable ATOMIC_REQUESTS per view.

A decorator is easier to apply to CBVs. Backwards compatibility isn't an
issue here, except for people running on a recent clone of master.

Fixed a few minor problems in the transactions docs while I was there.
Fixed fixtures testing failure

The failure was caused by generating the same warning from two tests.
The second time the same warning was raised it was swallowed by the
"once" simplefilter of warnings.
Lower the max length for a test field so that it works on MySQL.
Fix typo in redirect view docs
Fixed #20449 - Corrected test sensitivity to current working dir.
Fixed #20437 - marked a test-runner test as expected-fail under certain conditions.
Merge pull request #1181 from aaugustin/better-api-to-disable-atomic-requests

Changed API to disable ATOMIC_REQUESTS per view.
Merge pull request #1184 from alasdairnicol/reverse_view_docs_typo

Fix typo in redirect view docs
Fixed qs.values() regression when used in subquery
Fixed warnings in admindocs; refs #20126.
Fixed #20378 -- regression in GenericRelation on abstract model

When a GenericRelation was defined on abstract model, queries on childs
of the abstract model didn't work. The problem was in the way fields and
in particular field.rel was copied from models to their children.

The regression was likely caused by #19385. Thanks to Gavin Wahl for
spotting the regression.
Fixed #20278 -- ensured .get() exceptions do not recurse infinitely

A regression caused by d5b93d3281fe93cbef5de84a52 made .get() error
reporting recurse infinitely on certain rare conditions. Fixed this by
not trying to print the given lookup kwargs.