Fix no avatar not showing in user selection box
After upgrading to Ruby 2.5.3, we switched
URI.join in favor of
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.