Add paper to dashboard upon collaboration changes

References:
* https://www.pivotaltracker.com/story/show/77045390

Major changes:
* event stream now contains a user stream that can listen for specific events for a specific user
* generalizes event stream destroy action to work for any model (not just task)
* adds ability to refresh a user's event channels

--- AC + CT

Ah, `findTask` probably does the STI lookup we need...?