Attachment URL with non-/ relative root

The attachment URL was not working with relative_url_root not equal to '/'. I suggest this fix.

app/uploaders/attachment_uploader.rb

@@ -26,6 +26,10 @@ class AttachmentUploader < CarrierWave::Uploader::Base
     Gitlab.config.gitlab.relative_url_root + "/files/#{model.class.to_s.underscore}/#{model.id}/#{file.filename}"
   end
+  def url
+    Gitlab.config.gitlab.relative_url_root + '' + super unless super.nil?
+  end
+
   def file_storage?
     self.class.storage == CarrierWave::Storage::File
   end