Fix bug 812728: Rev dash - Add filter for changes from new users

Resolves [bug 812728](https://bugzilla.mozilla.org/show_bug.cgi?id=812728 "Provide filter on Revision Dashboard to show new users changes only").
Had a look at Django's ORM, but found no solution to write a QuerySet with GROUP BY and HAVING. So, I ended up with using a raw query.