improve clarity of "how to add release notes" message w/ a link to original commit

[ericphanson]

It could say “.. by retriggering registrator on your original commit” with a link back to that place

-@IanButterworth

This probably isn't super easy but would be quite nice. Needs to find the commit link from the registrator PR and thread it through to the message.

• ideally could handle non-github. At a minimum should not crash on non-github
• ideally could handle registrator webUI (juliahub). At minimum should not crash on non-comment-bot workflow