Allow former editors to be listed and skip the check on their group's participation

[deniak]

Fix #2003

[deniak]

@tidoust I've started implementing the patch to allow former editors to be listed and I'm forcing the <span class="former"> to start with the text '(until ' to avoid having these editors being listed just like the others.
That said, I can imagine some groups would want to provide different kind of reasons, e.g. https://www.w3.org/TR/mathml4/.
So I'm wondering if forcing that specific text is acceptable. It will still be an improvement compared to what we have today.

[tidoust]

Yes, I would personally only check that the span is not empty (after trimming) for the time being, to give groups enough leeway. It wouldn't check that the information is meaningful but we've been publishing specs for years without even looking into this, and I don't think anyone has willingly abused the system until now. It seems fine to trust people. As staff contact, what I really want is to trap the case when an active editor has not yet joined the working group.