Bug 1020348 - Added a tab to show aggregations on the signature report page.

@peterbe r? It's a follow-up to https://github.com/mozilla/socorro/pull/2144
But it is! It's right here: https://github.com/mozilla/socorro/blob/master/webapp-django/crashstats/signature/static/signature/css/signature_report.less#L27 (It was part of the previous PR. )