Fix approve button showing closed merge requests

What does this MR do?

Hides the Approve/Approve Additionally button when a merge request is closed or merged

Original "Approve additionally" MR, https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/4033

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Review
  • Has been reviewed by Frontend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

#4901 (closed)