Option to keep Job logs after the project is deleted

In the current specification, when a project is deleted, the job logs (#{ROOT_PATH}/gitlab-rails/shared/artifacts/#{disk_hash}/#{YYYY_mm_dd}/#{job_id}/#{job_artifact_id}/job.log) are also deleted.

On the other hand, an Ultimate customer is interested in an option to keep only the job log after the project is deleted since it may be used as an audit trail.