Try to make invisible space between usernames and messages intangible

We put an invisible space between usernames and messages to prevent them from running together internally even when the username is displayed in the margin with fringe between it and the message body. Point can be before the invisible space, which can make it appear to be on an "In reply to" link, but the link doesn't activate when the user presses RET because point is actually before the link. If we make the space intangible, it might fix this.