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.
Unused method argument - `filter`. If it's necessary, use `_` or `_filter` as an argument name to indicate that it won't be used. You can also write as `filtered_users(*)` if you want the method to accept any arguments but don't care about them.