[plug] Mailing list changes

[James Henstridge]

Hello everyone,

Today we migrated the mailing lists and website over to a new server.
Based on our testing, everything should be functional. But if you run
into issues, please email admin at plug.org.au.

While most things should look the same for now, there are a few user
visible changes with this upgrade:

1. The membership management app has moved from
https://www.plug.org.au/ugmm to https://ugmm.plug.org.au/. The old URL
will redirect to the new one, but it is possible your password manager
won't auto-fill.

2. We are running a slightly newer version of Mailman (although still
a 2.x release), which mitigates some issues with DMARC. This will
result in the "From:" address of some emails being rewritten to the
list address (something like "Original Sender via plug") if they have
a strict DMARC policy.

3. We are signing outgoing emails with a @plug.org.au "From:" address via DKIM.

These last two will hopefully reduce the chance of list emails being
marked as spam.

This migration has been a long time coming, with special thanks to
Benjamin Arntzen, Nick Bannon, and Tim White.

James Henstridge.