[1.6.x] Fixed #21428 -- editable GenericRelation regression

The GenericRelation refactoring removed GenericRelations from
model._meta.many_to_many. This had the side effect of disallowing
editable GenericRelations in ModelForms. Editable GenericRelations
aren't officially supported, but if we don't fix this we don't offer any
upgrade path for those who used the ability to set editable=True
in GenericRelation subclass.

Thanks to Trac alias joshcartme for the report and stephencmd and Loic
for working on this issue.

Backpatch of 0e079e4331a8be4dbd18d5e5776116330b0a5e61 from master.
[1.6.x] Fixed a regression caused by fix for #21428

On Python 3 sorting Fields mixed with GenericForeignKeys doesn't work
as GenericForeignKey isn't a subclass of django.db.models.fields.Field.

Refs #21428.

Backport of 326539f6a4 from master
Updated humanize translations and added release note.

Refs #21415.
[1.6.x] Fixed #21404 -- Added session.set_expiry() note to 1.6 release notes.

Thanks pwr for the suggestion.

Backport of ffdae5b66d from master
[1.6.x] Fixed #21240 -- Added 1.5 release note for OneToOneField/select_related change.

Thanks marcin at sokrates.pl.

Backport of ba63b9895b from master
[1.6.x] Fixed #20852 - Fixed incorrectly generated left quotes in docs.

Sphinx generates left single quotes for apostrophes after
code markup, when right single quotes are required. The
easiest way to fix this is just by inserting the unicode
character for a right single quote.

Instances of the problem were found by looking for
">&#8216;" in the generated HTML.

Backport of c33d1ca1d98003de29cdecb6080b52c5c52139bd from master.
[1.6.x] Added release note for TypedChoiceField coerce limitation

Thanks Elec for the report and Simon Charette for the review.
Refs #21397.
Backport of 4a00f132e0 from master.
[1.6.x] Added more internal links in the management command documentation.

Backport of bc0413cbc6fd472371e907f17303c5ddcc9b4642 from master.
[1.6.x] Added non-breaking spaces for humanize in Russian
[1.6.x] Fixed #21457 -- Allowed fixture file name to contain dots

Thanks Keryn Knight for the report.
Backport of 97ac22ebfc from master.
[1.6.x] Fixed typo in topics/http/sessions.txt.

Backport of 9348fc5628 from master
[1.6.x] Fixed the base class note in the Date Views docs.

Backport of 42197e31aa from master
[1.6.x] Added more examples to the get_object_or_404 documentation.

Backport of ebfa508fa32b27de6f3e2ca04c81b46408b0506e from master.
[1.6.x] Recommended setuptools in the reuseable app tutorial.

setuptools has merged with distribute.

Backport of b5eef8535a from master.
[1.6.x] Fixed #21472 -- Fixed inline formsets display when parent pk is 0

Thanks agale031176@gmail.com for the report.
Backport of fafb6cf049b from master.
[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