Fix the reply draft banner sticking around after a second publish.

Publishing a reply to a comment and then following it up with a second
reply would result in the reply banner sticking around, acting as if the
publish hadn't gone through.

This happened because signal handlers for the "publishing" and
"destroyed" events were registered twice on the first ReviewReply, which
would cause the state to mess up. We only really needed to register
these once.

Reviewed at https://reviews.reviewboard.org/r/5964/