Fix short timeago

What does this MR do?

Refactors previous implementation of gl.utils.shortTimeAgo

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

Reverts https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5225 with a better approach.

Why was this MR needed?

The previous implementation was working fine but the date was being updated every minute with the short format not being kept. This MR fixes that

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5225

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 style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together