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

In the code which existed before, we sent along only paper IDs. Now we include the `current_user` object. Is this intentional, or should we be sending something like `{ type: 'User', id: 123 }`?