Campaign update preview says closed/merged changesets will be updated

Created by: mrnugget

This is part of https://github.com/sourcegraph/sourcegraph/issues/8846 and is the extraction of this comment: https://github.com/sourcegraph/sourcegraph/issues/8846#issuecomment-597643803

When previewing a campaign update that includes merged/closed changesets, those are displayed under "To be updated":

Instead we introduce a "Unmodified" tab and list existing changesets there that will not be closed or deleted, but that are already closed or merged.