Added slash command to close an issue as a duplicate.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/31752

What does this MR do?

I introduced a /duplicate #issue_id slash command to mark an issue as a duplicate of another issue and close it.

A new icon for the duplicate action would need to be added for #24784 (closed) if an icon is needed.

cc @smcgivern

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

Why was this MR needed?

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary

• 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?

Closes #26372 (closed)