Internal improvements for members

Spined-off of !5566 (closed). The plan is as follow:

  • New Members::RequestAccessService: gitlab-org/gitlab-ce!6265
  • New Members::ApproveAccessRequestService: gitlab-org/gitlab-ce!6266
  • Improve Members::DestroyService: gitlab-org/gitlab-ce!6267
  • New AccessRequestsFinder: gitlab-org/gitlab-ce!6268
  • Make Member.add_user handle access requesters !6393 (merged)
  • Use Grape DSL to document Members API endpoints: gitlab-org/gitlab-ce!6269