API: Use Grape DSL to document access requests and members endpoints

Part of #21979 (closed), depends on gitlab-org/gitlab-ce!6267 and gitlab-org/gitlab-ce!6266.

Does this MR meet the acceptance criteria?

• 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 you do - rebase it please)
• Squashed related commits together