Convert EventStream Paper Streams to Users

### References
* https://www.pivotaltracker.com/story/show/82119744

### Features
* Accessibility class that takes a resource and finds which users are able to see it
* Each user gets only 2 channels: user and system. There are no more paper channels.
    - The user channel receives model creations/updates
    - The system channel receives model deletions
* Task generator updates to comply with the event stream and policy changes
* All serializers have knowledge of a "user" which means  all the if-then logic around whether a user exists or not when serializing data has been removed from the serializers.

### Implementation Notes
This is a rather large commit that may be a bit overwhelming at first glance, but focus your attention on:
* `subscriptions.rb` (which used to be known as `event_stream_subscriptions.rb`) which has been simplified to subscribe to all model changes in a common manner
* `event_stream.rb` which is responsible for taking a resource (i.e., `paper`), builds the json payload, and sends it along to the event_stream_connection
* `event_stream_connection.rb` (which used to be known as `event_stream.rb`) that has the sole purpose of talking to the go server in a pretty "dumb" fashion
* `accessibility.rb` is used by the event_stream and is responsible for answering the question, "given this particular resource (i.e., paper), who are all the users that are connected to it?".  It also passes all those users through the policy object for the resource to be absolutely sure should be allowed to see the data for the resource
* updates to all of the `policy` objects - the big change here is adding a `connected_users` method which has intimate knowledge of what users are connected to the resource

### Out of Scope
Please be aware that there are a bunch of existing event stream related pivotal cards that are out of scope for these changes. We wanted to highlight one particular issue from one of these related pivotal cards: 
 
* If a user is only associated to a paper through a single `paper_role` record and that `paper_role` is deleted, the paper does not disappear from the user's dashboard.  This will be fixed with https://www.pivotaltracker.com/story/show/82119774

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#### AC & CT
I actually agree with these somewhat.  `-> null` is a little more idiomatic for our current codebase, and we do try to use `this` instead of `@` when passing it alone.