Restore `get_id` of `ExtractsPath`

As mentioned by @DouweM in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5933/diffs#note_14352187, !5878 (merged) removed get_id from ExtractsPath breaking ProjectsController. While there is a quick fix !5933 (merged), the real fix would be to restore the overridden get_id. Otherwise @id is assigned twice.