[1.6.x] Correctly format missing Pillow/PIL exceptions messages. refs #19934

Backport of b9590a6935 from master.
Fixed #20943 -- Weakly reference senders when caching their associated receivers

Backport of e55ca60903 from master.
[1.6.x] Fixed a test failure introduced in f0bc2865ff.

`classobj` objects cannot be weakly referenced on Python 2.6
[1.6.x] Fixed #20820 -- Model inheritance + m2m fixture loading regression

Tests by Tim Graham, report from jeroen.pulles@redslider.net.

Backport of 1ed77e7782 from master
[1.6.x] Fixed invalid testing fixture

Backport of 86f4459f9e from master
[1.6.x] Fixed #20933 -- Allowed loaddata to load fixtures from relative paths.

Backport of 6e846f7627ecf0dc15053624a23bfbf47535972d from master.
[1.6.x] Documentation - Noted that OneToOneField doesn't respect unique.

Added OneToOneField to the list of model fields for which the unique
argument isn't valid. (OneToOneFields are inherently unique, and if
the user supplies a value for unique it is ignored / overwritten.)
[1.6.x] Fixed docstring typo, thanks minddust.

Backport of d3ed15b79d from master
[1.6.x] Fixed #20949 -- Typo #2 in docstring

Backport of 0073f1d94f from master
[1.6.x] Documentation -- Corrected error about Model.full_clean()

Although the ModelForm validation code was changed to call
Model.full_clean(), the documentation still said otherwise. The
offending phrase was removed.

Backport of bb011cf809 from master
[1.6.x] Fixed #20955 -- select_related regression

In cases where the same connection (from model A to model B along the
same field) was needed multiple times in a select_related query, the
join setup code mistakenly reused an existing join.

Backpatch of 8d65b6082c8bf5df25608d8733470879a8a61d7d.

Conflicts:

	django/db/models/sql/compiler.py
	tests/queries/tests.py
[1.6.x] Moved translator comment just above the target string

Backport of 8cd874298 from master.
[1.6.x] Made description of LANGUAGE_CODE setting more clear.

297f5af222bde02a7cdd005da2e4b00ec81801de from master.
[1.6.x] Fixed #20944 -- Removed inaccurate statement about View.dispatch().

Backport of bac4d03ce6 from master
[1.6.x] Documentation -- Clarified use of 'view' in test client introduction.

Backport of 2e926b041c from master
[1.6.x] Fixed #20922 -- Allowed customizing the serializer used by contrib.sessions

Added settings.SESSION_SERIALIZER which is the import path of a serializer
to use for sessions.

Thanks apollo13, carljm, shaib, akaariai, charettes, and dstufft for reviews.

Backport of b0ce6fe656 from master
[1.6.x] Typos introduced in 57c82f909b.
[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