Resolve "`Member.add_user`doesn't detect existing members that have requested access"
requested to merge 21983-member-add_user-doesn-t-detect-existing-members-that-have-requested-access-ee into master
This merge request allows Member.add_user to handle access requesters, so the temporary fix from https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/655 is not needed anymore.
Based on CE merge request: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6393 Includes the merge commit from !6266.