23718/backup rake task human readable

What does this MR do?

Add the human readable format to the backup tar file. From 1477317140_gitlab_backup.tar to 1477317140_2016_10_24_gitlab_backup.tar.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

#23718 (closed) issue

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

23718