x/build/cmd/gopherbot: add/remove label functionality can be confusing if original comment is deleted

[dmitshur]

gopherbot allows anyone to add or remove labels, as described at https://golang.org/wiki/gopherbot. Normally, it's possible to see who made the label add/remove change because of the comment requesting it. However, if that comment is deleted, it looks like gopherbot did it on its own, and it's not easy to find out why:

maintner tracks all events and can be helpful in situations like this, but this issue is about finding a better way.

This happened recently, see #35482 (comment) and #35755.