reintroduced unread entries tracking

Reintroduced the unread entries tracking for dialog conversations. A new site level activity setting is added called "'dialogue_trackreadentries" which if set (default is "1") will allow members of a course to see a total of conversations in each dialog not yet read. The user must also have $USER->trackforums set in their personal profile (forum tracking is a similar enough preference that it seemed ok to piggy-back on it)