list archives need porting

The list archives need to be ported to mailman.

We used to have a hand-rolled archiving script with majordomo lists. In the most
recent server move we reconstituted everything as mailman lists. Posts are now
quietly accumulating at http://lists.xiph.org/pipermail/ but this is not linked
from the usual http://xiph.org/archives/ page.

There are several parts to this.

 * an mbox version of all the old list archives needs to be generated so it can
be fed to mailman, so we have complete pipermail archives.

 * We need markup, ideally css, to make the mailman archive pages look like the
rest of the xiph.org site/the old archive pages.

 * We need to figure out how not break and references; i.e. keep old archive
pages at the same url; or symlink.