Fix no avatar not showing in user selection box

After upgrading to Ruby 2.5.3, we switched URI.join in favor of Gitlab::Utils.append_path. However, ActionController::Base.helpers.image_path can return a full URL if a CDN host is present.

Rails provides a way to generate the full URL using the asset path, but that doesn't appear to work because request is nil`.

Revert to the previous behavior to handle CDNs and relative URLs.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/56268