Consider bringing back v3 endpoint for OAuth endpoints
It seems a number of applications, including GitLab 10.8 itself (https://gitlab.com/gitlab-org/gitlab-ce/issues/47543), still uses the v3 API for OAuth applications.
We don't need to bring back the entire v3 API; rather just a passthrough for OAuth authentication.
/cc: @smcgivern, @fjsanpedro