Trigger custom events on comment add/remove/reply (git chery-pick from master branch)

Add custom discussion events and trigger them when a comment is added, removed or replied.
Also register these event as content-rules events in order to easily define custom content rules.
To be able to define useful content rules (send custom emails) also added custom string substitution adapters:
Comment Id
Comment Text
Comment User Id
Comment User Full Name
Comment User Email