Send out a notification to page watchers when a comment is made public

[Aaron Wells]

As described here: https://bugs.launchpad.net/mahara/+bug/1374674/comments/9

If you put a page on your watchlist, you will receive notifications when public feedback is placed on that page. However, you will not receive a notification if a person places private feedback which is later made public. Or if the page has comment moderation turned on.

It would be good to send a notification out to page watchers when private comments are made public.

To replicate:

1. Log in as User A.
2. Create a page. Allow comments, and make the page public.
3. Log in as User B.
4. Place the page on your watchlist.
5. Log back in as User A.
6. Place a comment on the page, marking the comment "private".
7. Once the comment has been posted, make it public.

Expected result: User B receives a notification about this new comment they can now see
Actual result: User B receives no notification.