Add the List-ID header to administrivia messages

Open username-removed-35210 requested to merge abompard/mailman:administrivia-headers into master

RFC 2919 states about List-Id: "This header SHOULD be included on all messages distributed by the list (including command responses to individual users), and on other messages where the message clearly applies to this particular distinct list."

It was not included in held message notifications before, I think it should be.

Fixes #164