Bugfix unread comments count

References:
* https://www.pivotaltracker.com/story/show/78659382
* https://www.pivotaltracker.com/story/show/80212178

Adding participants to a task is really clunky currently, partly because it is treated as a task update, which causes event stream to destroy the state of unread comment counts on the associated `litePaper`. This PR pulls the participation flow out to allow updates to the user channel.

Other goodies:
* Sending the participant email is no longer in `task#update`
* Fixes issue where participants on new message cards were not getting the email

 **AC & CT**