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

[timrogers]

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.