Add cron job to clean carrierwave's temfiles
Description
Carrierwave
gem creates tempfiles every time when a file is being uploaded.
!9431 (merged) brings an ability to cancel file uploading.
This feature can increase a number of tempfiles.
Proposal
Add a cron job for calling CarrierWave.clean_cached_files!
as discussed: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/9431#note_23913161