(Un)subscribe feature for comments

[ain]

It's desirable to be able to subscribe and unsubscribe from the emails that are being sent on a comment.

I'd suggest adding Subscribe or Unsubsribe link to the Expand all | Collapse all block, so that it becomes Expand all | Collapse all | Subscribe or Expand all | Collapse all | Unsubscribe depending on user's current preference. Default would follow the current logic of being subscribed if previously commented.

[steveyken]

:+1: