mailman: set URL patterns to be https by default

Set DEFAULT_URL_PATTERN and PUBLIC_ARCHIVE_URL to patterns that speak
HTTPS by default.

After setting this, one needs to manually fix list URLs as outlined at:

  http://tinyurl.com/osqbo2y

In our case, that means doing the slightly nasty hack of:

1) update the config as in this diff (i.e. with https)
2) run `withlist -l -a -r fix_url`
3) temporarily revert the config to contain http URLs
4) run

       list_lists -b |
       grep ^xp- |
       xargs -n1 -IREPL withlist -l -r fix_url REPL -u lists.dataexpeditions.org

5) re-revert the config to contain https URLs