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 to spike out a pattern using `Paper` as an example.  Other resources could emulate this same pattern.
* This is a step toward allowing an easier, larger refactor of `EventStream` and does not include any major refactor to EventStream in this PR.  Baby steps.

# Feedback on approach welcomed!

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