Allow a configuration option to send out "page update" notifications using the authors email address

This is related to CONF-5792.

The idea is that if I have a watch on a page, then I would like to get update notifications from the author of the change. That would allow me to directly reply to the email that I get in order to give them some feedback.

The concern in CONF-5792 was that for a public facing confluence instance, spammers would set a bunch of watches in order to harvest email addresses. That concern completely makes for externally facing instances.

On the other hand, this is not a concern for internal only systems. What ends up happening on our instance several times per week is that someone will respond to a "page was updated notification", which ends up going to a black hole.

When a user gets a notification of a change by a specific user, it is just natural to be able to send them immediate feedback.