[1.6.x] 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.

Backport of b6a6cf4ab791b092e18ab16537aa88dd7ae9bc36 from master.
[1.6.x] Fixed #21431 -- GenRel->FK list_filter regression in admin

Report, analysis and tests from stephenmcd.

Backpatch of 752d3d70da6291039f33781a0a2ef6f3b7c5fcb5 from master.
[1.6.x] Fixed #21405 -- Prevented queryset overwrite in BaseModelAdmin

Thanks guido@20tab.com for the report and Tim Graham for the
analyze.
Backport of 1718b5256c from master.
[1.6.x] Fixed #21487 -- Session cannot store tzinfo instances anymore.

Thanks filipp for the report.
[1.6.x] Fixed sentence structure in docs/internals/contributing/triaging-tickets.txt

Backport of 4a54b69e88 from master
[1.6.x] Fixed #21443 -- Cannot show debug info on PY3's importlib

Thanks productions@zaziork.co.uk for the review.
Backport of 18185724e6 from master.
[1.6.x] Noted that localmem is the default cache.

Backport of c456ea4ec8 from master
[1.6.x] Improved docs for ModelFormSet.clean().

Backport of 033b26173b from master
[1.6.x] Removed obsolete deprecation notes.
[1.6.x] 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.
[1.6.x] Fixed #21486 -- Prevented settings config in signal connection

This was particularly problematic in the chain get_wsgi_application
-> db.connections import -> signal connection -> settings configuration.
Thanks Jon Dufresne for the report.
Backport of 42fef294464 from master.
[1.6.x] Fixed #21490 -- Fixed custom admin URL reverse example.

Thanks glarrain for the report.

Backport of bfe7377adb from master
[1.6.x] Fixed typo in previous commit; refs #21490.

Backport of 03bc0a8ac5 from master
[1.6.x] Fixed #21507 -- Corrected default widget information for FileField

Backport of 70e61310d6 from master
[1.6.x] Fixed #21355 -- try importing _imaging from PIL namespace first.

Backport of 5725236c3e from master
[1.6.x] Fixed #21448 -- Fixed test client logout with cookie-based sessions

Thanks Gunnar Scherf for the report and the suggested patch.
Backport of 384816fcc from master.
[1.6.x] Fixed #21515 -- Corrected example of template.Context in documentation.

Thanks to trac user oubiga for the report.

Backport of 077af42139db84d88f293ab5eadc989a9169dce1 from master.
[1.6.x] Fixed #21496 -- Fixed crash when GeometryField uses TextInput

Thanks Rhett Garber for the report and initial patch.
Backport of 34b8a3855 from master.
[1.6.x] Fixed #21535 -- Fixed password hash iteration upgrade.

Thanks jared_mess for the report.

Backport of fddb0131d3 from master
[1.6.x] Fixed incorrect type for max_length.

Backport of 3b60ffa334 from master