Check if a merge request is approved when merging from API or slash command

What does this MR do?

It checks if a merge request was approved when merging using slash command or from API.

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

Why was this MR needed?

We don't check if an MR is approved when merging from API or merge slash command

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 #2052 (closed)