MDL-40062 mod_forum: remove subscribeall/unsusbcribeall add_to_log

Now individual subscriptions will fire events, there is no need for a
catch all.
MDL-40062 mod_forum: remove read tracking log entries

Read tracking individual log entries do not provide enough reliable
information, as the posts can be marked as read automatically after
emailing or an expiry period.
MDL-40062 mod_forum: course_searched event
MDL-40062 mod_forum: Add forum_viewed event
MDL-40062 mod_forum: read tracking events