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 #21386 -- admindocs depend on sites framework