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.
Fixed #21217 -- Avoid connecting `(pre|post)_init` signals to abstract senders.