Optout

adds model and methods in conversation:

- opt_out(participant)
- opt_in(participant)
- has_subscriber?(participant)

current setup is all conversation's recipient are opt in by default, then they can opt out if they desire (this way they wont receive emails anymore whenever there is one more message related to this conversation).

What is not included (yet): opt in a conversation when not a participant (use cases are pretty rare I guess).