Trigger custom comment events on comment add/remove/reply

Conflicts:
	plone/app/discussion/interfaces.py
Use commented object as context instead of comment within event trigger

- In order this to work with plone.contentrules use commented object as
  the main context within custom discussion events and add comment as the
  second parameter.
Update change log

Conflicts:
	CHANGES.rst
Add custom content rules string substitutions

- Add content rules string substitutions for comment:
  - id
  - text
  - author email
  - author user name
  - author full name