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.
There was some confusion on the mentorship list as to whether 'needs tests'
meant that a given ticket might not need tests, as opposed to flagging
a specific patch as still needing tests.
This commit seeks to clarify that confusion.
Now that DDN is gone, I felt it was worth some extra language
about what "accepted" means, and qualify what it means to be "safe"
to start writing a patch.
