Return 202 with JSON body on async removals on V4 API

What does this MR do?

Basically return 202 and proper JSON on async removals on:

• DELETE /projects/:id/repository/merged_branches
• DELETE /projects/:id

---

• Changelog entry added
• 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 #3874 (closed)