Messaging for mailman3 transition

One of the next big infrastructure transitions is to move us off of SourceForge to our own self-hosted mailman3 instance, and I need to start putting out some announcements about this change.

Normally this would be no big deal, but the complicating factor is that I can't download the existing mailing list subscribers, as Sourceforge now makes this info inaccessible. So, unfortunately it looks like we're going to have to just encourage subscribers on the old lists to subscribe to the new ones.

So, I could use some help in drafting a few announcements, first that the change is coming, second that it has happened, and a third for a handful of lists that we're closing. Critically, the messaging should get people motivated to register for the new lists.

https://gitlab.com/inkscape/vectors/content/blob/master/articles/mailman3%20email%20messaging/Messaging_for_mailman3_transition__46.md

• Subject: [ANNOUNCE] This mailing list is moving soon!
• Subject: [IMPORTANT] This mailing list has moved to lists.inkscape.org!
• Subject: [ANNOUNCE] This list will be closing soon
• Subject: [IMPORTANT] This list has closed