New `/duplicate` quick action

What does this MR do?

Follow-up to https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/10391 - thanks, @archytaus!

Why was this MR needed?

This MR for CE was a blocker for the GNOME project in their migration to GitLab and was decided that it meets the criteria for a CE feature.

Screenshots

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• 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?

Closes #26372 (closed).