MDL-40062 mod_forum: replace prune post event

Splitting a post actually means new discussion created
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: add discussion_created event
MDL-40062 mod_forum: add discussion_deleted event
MDL-40062 mod_forum: convert discussion moved add_to_log
MDL-40062 mod_forum: subscription created event
MDL-40062 mod_forum: subscription_deleted event
MDL-40062 mod_forum: read tracking events
MDL-44062 mod_forum: subscribers_viewed event
MDL-40062 mod_forum: convert userreport log entry to event
MDL-40062 mod_forum: post_created event
MDL-40062 mod_forum: post_deleted event
MDL-40062 mod_forum: post_updated event
MDL-40062 mod_forum: add event unit tests