Enable fast_destroy_uploads flag in production

There is a new feature - async deletion of uploads (https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/20977), this is behind feature flag and disabled. To make sure this feature works properly, I'd like to do following steps on production:

• run rake gitlab:cleanup:remote_upload_files on prod to get current number of orphaned files
• enable fast_destroy_uploads feature and wait for 1-2 days
• run rake gitlab:cleanup:remote_upload_files and compare number of orphaned files - if there is significant increase of orhpaned files, it might be sign that uploads deletion doesn't work as expected

Running this rake task should not cause dramatic IO load on prod server (as most of checking should be done in object store) and it's expected to run probably few hours - https://gitlab.slack.com/archives/C101F3796/p1547136320445300

I set due date to 21st because based on slack discussion that week would be preferred to do this instead of doing it now

/cc @skarbek @ahanselka