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.
Merge remote-tracking branch 'upstream/master'