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.
This formally describes our policies on reporting, notification and
disclosure of security issues, and provides a detailed explanation of
our full security-response process, for reference purposes.
