fixes bug 794548 - display "exploitability" rating in UI to logged-in users

@rhelmer r?

(cc @AdrianGaudebert, @twobraids)

Instead of making `processed` vs. `unredacted` a parameter on the `ProcessedCrash` model I introduced another model. I think they are too different to try to "merge" into one thing. Also, the unredacted one shouldn't even go near the public API due to it's sensitive nature. 

I still don't think I understand why the middleware does a bunch of business logic decisions about what data you deserve to get when that general source of that decision is done in Django. @twobraids?