Fix Vue warnings about string idKey in Related Issues

What does this MR do?

Fix Vue warnings about string idKey in Related Issues

[Vue warn]: Invalid prop: type check failed for prop "idKey". Expected Number, got String.

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

• Needs to be picked in the CE->EE MR, https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/2178

Why was this MR needed?

We now fail on Vue warnings, https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/12163/

Does this MR meet the acceptance criteria?

• 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?