Revert !1088 "Stop using gitlab-shell hooks -- but keep using gitlab-shell config"

This reverts merge request !1088 (merged).

The reason is that the new hook location broke CI for gitlab-workhorse, and I don't see an easy way out of that. gitlab-workhorse!373 (closed)

I think the breakage is specific to tests / CI. The hooks make requests across projects, and in CI, we often don't have a complete enough GitLab installation. In the workhorse example, we are not running an instance of gitlab-rails. So I don't think we necessarily have to propagate this revert MR into a gitlab-ce stable branch.

However, it is not good to have broken CI in workhorse, and by reverting this we solve that immediate problem until we can come up with a real solution.

This temporarily addresses gitlab-workhorse#211 (closed)