Get rid of redundant watchlist adds, for users who would receive feedback notifications anyway

[Aaron Wells]

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

In Bug 1374674, we made it so that when you comment on a page, that page is added to your watchlist. However, we set it so that this does not happen if you are the owner of the page, because then it's redundant (you already receive notifications about feedback to the page).

It would be good to extend that redundant-watchlist-prevention to site pages, group pages, and institution pages as well. I.e., if you are a site admin and you comment on a site page, we don't add it to your watchlist.