[Feat]: Pivotal Tracker import should add PT Story ID as a comment

[st-kep]

When importing a Pivitol Tracker export, it would be helpful if the importer added the orginal PT story number (id) as a comment.

i.e. Original Pivotal Tracker Story Number 123456

At least for us, the Pivotal Track story number is referenced in code comments to explain the change and to point maintainers where to go for more information on the change.

Looking through the code, it looks like this would be an easy feature to implement and one we may implement ourselves prior to final importing. Code to add custom comments is already being used to add review information and the story id ("external_id") is referenced when determining if there is a file to attach.