Trigger custom events on comment add/remove/reply

- 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