Fixed #21469 -- Allow set objects in Meta.unique_together.

Thanks to Tim for the review.
Fixed #21458 -- Made check_for_language more resistant to malformed input.

Thanks to Sergey Sorokin for the report and to Bouke Haarsma for the review.
Fixed #21472 -- Fixed inline formsets display when parent pk is 0

Thanks agale031176@gmail.com for the report.
Refs #8261 -- Fixed regression introduced by fd219fa.

ModelAdmin displayed the "View on site" link even if the Model didn't
define the `get_absolute_url()` method.
Merge pull request #1954 from loic/ticket8261

Refs #8261 -- Fixed regression introduced by fd219fa.
Fixed #21431 -- GenRel->FK list_filter regression in admin

Report, analysis and tests from stephenmcd.
Removed isinstance(RelatedObject) checks from admin.utils
Fixed #21427 -- Clearly state integer field value ranges in docs

Added an explicit mention of the exact value ranges for integer type
fields that are safe in all databases supported by Django. Also, put
all value numbers inside double ticks.
Fixed #21479 -- Favor 'migrate' over 'syncdb' in the docs.
Fixed #21476 -- Cache tests make an incorrect use of `HttpRequest`

Using `django.test.client.RequestFactory` solves the problem and cleans up all
the `get_request` mess.
Made minor optimizations to admin_ordering tests
Fixed #21405 -- Prevented queryset overwrite in BaseModelAdmin

Thanks guido@20tab.com for the report and Tim Graham for the
analyze.
Updated admindocs to use class-based views

Thanks Bouke Haarsma for the review.
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.