Bug 1020348 - Added new signature-based report with a simple data tab.

I think @peterbe, @lonnen or @rhelmer can review this. It's django only, no back-end changes. 

Note that this is a first version that I am sending now for the sake of smaller PRs. The page only shows a list of crash reports at the moment, but it should work fine. It is not linked from anywhere yet, and is hidden behind a waffle switch, so it's safe to release it as is. I'm working on adding two other tabs asap, one for aggregations and the other to list comments.