When `bbs2gh migrate-repo` is interrupted, cancel any in progress Bitbucket Server exports

When you start a Bitbucket Server export with `gh bbs2gh migrate-repo`, you might want to stop this export - for example if you discover that you exported the wrong repo, it is taking too long or your Bitbucket Server's resources are getting consumed too quickly.

When `migrate-repo` is interrupted during the Bitbucket Server export process, we should cancel the running export on the Bitbucket Server using the [REST API](https://developer.atlassian.com/server/bitbucket/rest/v807/api-group-system-maintenance/#api-api-latest-migration-exports-jobid-cancel-post).