Keep merged and closed changesets in campaign to retain change log

Created by: eseliger

We currently drop all changesets from a campaign where we don't have a new changeset spec. That means when something got fixed upstream by merging one of the changesets, when I reapply that merged changeset will disappear from the campaign (because my search didn't return this repo anymore).

I think we should investigate if we can keep merged/closed changesets (and potentially also the ones we close).