MDL-40062 mod_forum: remove invalid add_to_log() calls

* debugging add_to_log() calls - the Moodle log is not the
  place for these kind of messages

* subscribeall/unsusbcribeall - now individual subscriptions
  will fire events, there is no need for a catch all.

* 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: convert viewed events

* course_module_instance_list_viewed
* forum_viewed
* course_searched
* userreport_viewed
MDL-40062 mod_forum: discussion events

* discussion_created
* discussion_deleted
* discussion_moved
* discussion_viewed
MDL-40062 mod_forum: subscription events

Events for forum mail subscriptions

* subscription_created
* subscription_deleted
* subscribers_viewed
MDL-40062 mod_forum: read tracking events

* readtracking_enabled
* readtracking_disabled
MDL-40062 mod_forum: post events

* post_created
* post_updated
* post_deleted

Prune post (split discussion) has been converted to multiple events as
splitting a post actually means new discussion created.
MDL-40062 mod_forum: add event unit tests