Fixed #20867 -- Added the Form.add_error() method.

Refs #20199 #16986.

Thanks @akaariai, @bmispelon, @mjtamlyn, @timgraham for the reviews.
Removed Form._errors from the docs in favor of the add_error API.
Fixed an unescisarily gendered pronoun in a docstring
Enabled makemessages to support several translation directories

Thanks Rémy Hubscher, Ramiro Morales, Unai Zalakain and
Tim Graham for the reviews.
Also fixes #16084.
Closed file after reading pot file

Faulty line was introduced in 50a8ab7cd1.
Fixed comment typos.
Documented the house style for gender neutral pronouns.
Removed gender-based pronouns per [c0a2daad78].
Fixed #21358 -- Allowed runserver on non-English locales

Thanks svartalf for the report.
Refs #21230 -- removed direct settings manipulation from template tests
Fixed #21535 -- Fixed password hash iteration upgrade.

Thanks jared_mess for the report.
Fixed incorrect type for max_length.
Fixed #21543 -- Removed base_dir attribute in StaticFilesHandler.

This code seems to be an artifact of AdminMediaHandler removed in [5c53e30607].
Fixed #15619 – Added CSRF protection in the logout view.

Logout is now performed only for POST requests.
GET requests return confirmation page.

Based on patches from ashchristopher and vzima.