Fixed #21209 -- .po file path comments on Windows.

Literals from source files with Django template language syntax don't
have a '.py' suffix anymore.

Also, the '.\' prefix is preserved to respect GNU gettext behavior on
that platform.

Refs #16903.
Fixed tests breakage from last commit.
Made skipping of symlink-related makemessages test smarter.

os.symlink() is available starting with Windows Vista but can fail at
runtime.
Made test for issue 19552 compatible with Windows.

Take in account platform path separator. Refs #19552.
Fixed #21129 -- Prevented admin filter params modifications from throwing an exception.

Thanks Tuttle for the report.
Fixed #21122 -- Improved clean up of test temp directory on Windows

- Fixed test that didn't close the files it opened
- Caught and handled exception when temp directory cannot be removed
Clarified session replay attack differences with cookie backend.
Changed the doc to use gender-neutral pronouns when possible.
Fixed #10913 -- Documented how related_name affects QuerySet filtering

Thanks neithere for the suggestion.
Initial version of MigrationOptimizer and tests
Fixed #16919 -- Passed user to set_password_form in GET requests.

Thanks Jaime Irurzun for the report and initial patch and
ejucovy for the test.
Fixed #21165 -- Fix test for syndication feed timestamp field on Windows.

Thanks Michael Manfre for the report, Raphaël Barrois for the patch and
Claude Paroz, Aymeric Augustin for the reviews.

Refs #7936.
Fixed #19182 -- Fixed ModelAdmin.lookup_allowed to work with ('fieldname', SimpleListFilter) syntax.

Thanks gauss for the report.
Fixed #21216 -- Allow `OneToOneField` reverse accessor to be hidden.
Fixed #19277 -- Added LocaleMiddleware.response_redirect_class

Thanks ppetrid at yawd.eu for the suggestion.
Fixed #21218 -- Typo on docs/howto/upgrade-version.txt

Thanks ryan at ryangallen.com for the report.
Fixed #19321 -- Allowed redirect middleware HTTP responses to be overridden.

Thanks Melevir for the suggestion.
Fixed #21213 -- Added docs for Django's mailing lists.

Added docs/internals/mailing-lists.txt documenting the use of django's
mailing lists. All references across docs changed to point to this page.

The referencing makes use of substitution because there's no way to make
a :ref: link in a non-inline fashion in Sphinx. It also makes use of
rst_epilog Sphinx conf for making this substitutions across all the
docs.
Fixed #6681 -- Don't break docutils when rendering reStructuredText.

Don't set a global default interpreted role function for reStructuredText.
Instead, use the `default-role` directive to change the default only within
the `parse_rst()` function.

Thanks Malcolm Tredinnick for the report.
Fixed #11277 -- Hid labels of fields with HiddenInput widget in admin forms.

Thanks bartTC for the report.