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