Don't pass a current user to Member#add_user in LDAP group sync
requested to merge ee-1159-allow-permission-check-bypass-in-approve-access-request-service-ee into master
See the individual commits for explanation of the changes.
This is part of the fix for gitlab-org/gitlab-ee#1159.
CE backport for the change in Members::ApproveAccessRequestService is at gitlab-org/gitlab-ce!7168.
/cc @stanhu