Paper role participation event stream

### References
* https://www.pivotaltracker.com/story/show/82119774

### Major Changes
* Event stream now sends the object itself (rather than just the id) to allow for scoped deletions. (e.g Deleting a `PaperRole` could mean we need to "delete" a `Paper` from the user's client store).
* The submitter of the paper has moved from `paper.user` to `paper.creator`
* Participations now create a paper role of participant, and are cleaned up upon the deletion of the last participation.
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