This adds the rake task rake
gitlab:cleanup:orphan_job_artifact_files. This rake task cleans all
orphan job artifact files it can find on disk.
It performs a search on the complete folder of all artifacts on
disk. Then it filters out all the job artifact ID for which it could
not find a record with matching ID in the database. For these, the
file is deleted from disk.
Calling `Fog::Storage::AWS::Directories#get` requires the
ListAllMyBuckets permission, but we can avoid that extra query and
permission by initializing the directory with a specific bucket:
https://stackoverflow.com/a/12288581/1992201
