The frontend should use the v3 API for now
We are adding a new version of the API, v4. As this is the default version, we include it in our gon variables: https://gitlab.com/gitlab-org/gitlab-ce/blob/v8.17.1/lib/gitlab/gon_helper.rb#L4
Which means our JS code uses it: https://gitlab.com/gitlab-org/gitlab-ce/blob/v8.17.1/app/assets/javascripts/api.js#L145
We didn't mean to do that, I don't think, as we're deliberately introducing some breaking changes. While we should migrate to v4 in 9.1, now is not the time, and we should make this use the v3 API for now.