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