Fixed #21460 -- Reenabled proper template precedence in find_template

Refs #20806. Thanks Unai Zalakain for the review.
Fixed missing admindocs' site_header
Merge pull request #1877 from Bouke/patches/4

Fixed missing admindocs' site_header, refs #21293
Fixed sentence structure in docs/internals/contributing/triaging-tickets.txt
Fixed #21487 -- Session cannot store tzinfo instances anymore.

Thanks filipp for the report.

Forward-port of 1eddca0a from stable/1.6.x.
Fixed some flake8 issues
Fixed #21491 -- Removed documented workaround for a known issue.

The issue was that two M2M hidden reverse managers
(related_name ending with a '+') could clash with each other.

Refs #21375 and #15932. Thanks Baptiste.
Merge pull request #1965 from loic/ticket21491

Fixed #21491 -- Removed documented workaround for a known issue.
Fixed #21443 -- Cannot show debug info on PY3's importlib

Thanks productions@zaziork.co.uk for the review.
Removed unused import in docs/topics/http/sessions.txt
Noted that localmem is the default cache.
Improved docs for ModelFormSet.clean().
Removed information about releases < 1.7.
Merge pull request #1973 from haikoschol/master

Removed information about releases < 1.7.
Fixed #21445 -- Clean up misuse of null in quickElement.

Thanks to trac user parsch for the report.
Fixed #21012 -- New API to access cache backends.

Thanks Curtis Malony and Florian Apolloner.

Squashed commit of the following:

commit 3380495e93f5e81b80a251b03ddb0a80b17685f5
Author: Aymeric Augustin <aymeric.augustin@m4x.org>
Date:   Sat Nov 23 14:18:07 2013 +0100

    Looked up the template_fragments cache at runtime.

commit 905a74f52b24a198f802520ff06290a94dedc687
Author: Aymeric Augustin <aymeric.augustin@m4x.org>
Date:   Sat Nov 23 14:19:48 2013 +0100

    Removed all uses of create_cache.

    Refactored the cache tests significantly.

    Made it safe to override the CACHES setting.

commit 35e289fe9285feffed3c60657af9279a6a2cfccc
Author: Aymeric Augustin <aymeric.augustin@m4x.org>
Date:   Sat Nov 23 12:23:57 2013 +0100

    Removed create_cache function.

commit 8e274f747a1f1c0c0e6c37873e29067f7fa022e8
Author: Aymeric Augustin <aymeric.augustin@m4x.org>
Date:   Sat Nov 23 12:04:52 2013 +0100

    Updated docs to describe a simplified cache backend API.

commit ee7eb0f73e6d4699edcf5d357dce715224525cf6
Author: Curtis Maloney <curtis@tinbrain.net>
Date:   Sat Oct 19 09:49:24 2013 +1100

    Fixed #21012 -- Thread-local caches, like databases.
Fixed #21488 -- Multiple locales treatment in i18n commands.

Removed multiple locales separated by commas variation (that wasn't
working as documented) in favor of simply allowing use of the
``--locale``/``-l`` options more than once for ``makemessages`` and
``compilemessages``.

Thanks Romain Beylerian for the report and Claude, Simon for their help.

8750296918072c97a51f6809d19828ce3f1b8d40 from stable/1.6.x.
Allowed running the test suite without memcached (!)
Simplified iteration in HTTP response objects.

Fixed #20187 -- Allowed repeated iteration of HttpResponse.

All this became possible when support for old-style streaming responses was
finally removed.
Fixes #21412 -- Better error message for messages.add_message

Problem were users calling messages.debug/info/* with a wrong argument
and getting the error "You cannot add messages without installing
MessageMiddleware"

Thanks to trac-user merb for the report.