Don't close already closed, detached changesets

Created by: eseliger

When a changeset is not attached to the campaign, we don't want to close it again. For a campaign that I had which a couple of time rolled over the branch name (so close all changesets and create new ones), this caused quite a few close operation calls to be made, and I think this is unnecessary, given it's been detached before already. We could've also added this to the store method, but then in the resolvers I wouldn't be able to get the matching changeset anymore, which I would like to get for visualizing close events in the future.

Closes https://github.com/sourcegraph/sourcegraph/issues/16148