Fixed #14549 - Removed restriction of single FKs on intermediary tables

Thanks to Loic Bistuer for review. Minor changes to error messages
done by committer.
Fixed #22207 -- Added support for GenericRelation reverse lookups

GenericRelation now supports an optional related_query_name argument.
Setting related_query_name adds a relation from the related object back to
the content type for filtering, ordering and other query operations.

Thanks to Loic Bistuer for spotting a couple of important issues in
his review.
Fixed issues and added new scripts to urlify.js

Added the following scripts:
* Lithuanian (thanks to petraszd for the patch)
* Serbian (thanks to offy)
* Azerbajani (thanks to Ali Ismayilov)

Fixed the following issues:
* In Polish character map, some uppercase letters were
  converted to a lowercase ascii equivalent.
* The letter Y with a diaeresis had a lowercase version but no uppercase.

The code was also simplified and cleaned up:
* Use of `var` statements everywhere
* Systematic use of semicolons
* Proper looping over arrays and objects.

Thanks to @oinopion for his help in getting the javascript
into proper shape.

Fixes #8561, #11035.
Added --previous flag to msgmerge command used by makemessages

Also took the opportunity to slightly refactor gettext options
so as to ease customization by subclassing the command.
Thanks Michal Čihař for the report and initial patch.
Fixed #22185 -- Added settings.CSRF_COOKIE_AGE

Thanks Paul McMillan for the review.
Reworked ErrorDict.as_json() to prevent unnecessary serialization/deserialization step.

Thanks @apollo13 for the suggestion. Refs #17413.
Fixed #22183: Don't make a table for M2Ms with through=
Revert "Fixed #22183: Don't make a table for M2Ms with through="

This reverts commit 1562b9896f8f614ef40fd032b1ec777280b377c1.
Fixed #22204: Bad circular-dep-breaking if more than one per run
Don't allow really long migration names
Fixed #22070 -- Changed verbose_name for apps in django.contrib to use title case

Thanks bendavis78 for the report.
Fixed typo in docs/ref/checks.txt
Fixed #22141 -- Added a spelling checker for documentation.
Fixed #21863 -- supplemented get_lookup() with get_transform()

Also fixed #22124 -- Expanded explanation of exactly what is going on in
as_sql() methods.
Fixed #22034 -- Added a specific set of relation checks for GenericInlineModelAdmin.

Thanks to jwa for the report.
Fixed #21188 -- Introduced subclasses for to-be-removed-in-django-XX warnings

Thanks Anssi Kääriäinen for the idea and Simon Charette for the
review.
Avoided changing raw DeprecationWarning filter behavior

Refs #21188. Now pure Python DeprecationWarning visibility should
be back to Python defaults.
Added test about error filterwarnings

Refs #20083.
Avoided modifying current environment in admin_scripts tests
Fixed #21092 -- Ensured admin_scripts tests are run with warnings off