[1.6.x] Fixed #20961 -- Fixed HttpResponse default empty content

Thanks epandurski at gmail.com for the report.
Backport of f4e980456 from master.
[1.6.x] Updated instructions for running contrib tests.

Backport of 08e7a64369 from master
[1.6.x] Fixed typo in docs/topics/conditional-view-processing.txt

Backport of 5fd2c979cb from master
[1.6.x] Fixed #20981 -- Noted the default value of disable_existing_loggers.

Backport of 095643e691 from master
[1.6.x] Minor spelling correction in ModelForms docs

Backport of 2fbf949760 from master
[1.6.x] Fixed #20984 -- Stopped decoding bytes in sqlite3 adapter on Python 3

Thanks lvella at gmail.com for the report.
Backport of 169637649 from master.
[1.6.x] Fixed #16433 -- Fixed a help_text/read only field interaction that caused an admin crash.

Thanks chris at cogdon.org for the report and admackin for the patch.

Backport of af953c45cc from master
[1.6.x] Fixed #14786 -- Fixed get_db_prep_lookup calling get_prep_value twice if prepared is False.

Thanks homm for the report and Aramgutang and lrekucki for work on
the patch.

Backport of f19a3669b8 from master
[1.6.x] Added links to file docs.

Backport of d72f83c410 from master
[1.6.x] Removed stale add_q() comment

Backport of 13be3bfef1 from master
[1.6.x] Fixed #20988 -- Added model meta option select_on_save

The option can be used to force pre 1.6 style SELECT on save behaviour.
This is needed in case the database returns zero updated rows even if
there is a matching row in the DB. One such case is PostgreSQL update
trigger that returns NULL.

Reviewed by Tim Graham.

Refs #16649

Backport of e973ee6a9879969b8ae05bb7ff681172cc5386a5 from master

Conflicts:
	django/db/models/options.py
	tests/basic/tests.py
[1.6.x] Set 'bidi' Urdu property to True

Refs #20454.
Backport of e4a67fd90 from master.
[1.6.x] Fixed #20999 - Allow overriding formfield class with choices, without subclass restrictions.

Refs #18162. Thanks claudep and mjtamlyn for review.

Backport of 7211741fc5d50425 from master.
[1.6.x] Fixed typo in docs/ref/models/options.txt

Backport of a89c856a7a from master
[1.6.x] Fixed copy/paste error in measurement docs

Backport of e87997dd33 frmo master.
[1.6.x] Made the doc about translating string literals in templates more prominent.

Backport of 9885f07757 from master
[1.6.x] Fixed #20998 -- Allow custom (de)serialization for GIS widgets

Thanks Mathieu Leplatre for the report and the initial patch.
Backport of 102f26c92 from master.
[1.6.x] Fixed gis test to run on non gis-enabled settings

Refs #20998.
Backport of 973502c0 from master.
[1.6.x] Fixed syntax error in OSMWidget

Backport of 6ecbac21a from master.
[1.6.x] Prevented rendering attrs to be squashed in OSMWidget

Backport of b6889c68d7 from master.