added restricted flag to preprocessor; purpose: limit who can see a chunk to the authors and assigned reviewers
Minor fixes / stop gaps

There is an error in notifications/models.py due to the multiple
authors change. I added a return statement right before the
offending line so that the bug doesn't effect the comment-posting UI.

Other fix is to fetch the reviewers for a submission when doing access control.
The Chunk.sorted_reviewers method does not include reviewers who are assigned
to the submission containing that chunk. This should probably be changed.
Merge branch 'master' of https://github.com/uid/caesar-web