a8n: Allow closing of changesets on codehosts when deleting campaign
Created by: mrnugget
This PR contains the mirror implementation of #7127 for the deleteCampaign
mutation. When specifying closeChangesets
the mutation will close the changesets on the codehost after deleting the Campaign on Sourcegraph.
It also contains a refactoring of the closeCampaign
mutation: I moved the logic into the a8n.Service.CloseCampaign
method so that it's closer to the implementation of DeleteCampaign
and we don't have more business logic lying around in the resolver.