Single Streaming [DO NOT MERGE]

### References
* https://www.pivotaltracker.com/story/show/81129650

### Major Changes
* Example of how paper event_stream actions can be pushed down the User channel, rather than the Paper channel
* Addition of a `Accessibility` class which answers the question:  "What users have access to this resource (i.e., paper)?"
* `Accessibility` class ensures that security is maintained by using the matching resource policy (i.e, PaperPolicy) to determine if the user should have access to the data
* Policy has a notion of `connected_users` which is the superset of all users that have access to its resource

### Important Notes
* This is simply a step toward allowing an easier, larger refactor of `EventStream`.  Baby steps.


# Feedback on approach welcomed!

--- AC + CT
There obviously can be much more done with improving event_stream, but this PR is more about figuring out what data can be serialized and is it safe to send.  Once this is done, it will make it easier to clean up and simplify the event stream.