Use the new stats layout in budget stats

So now stats by gender and age are replaced by shared participation
stats (which also includes stats by district), reusing the code already
used in poll stats, and advanced statistics (which used to be at the top
of the page) are now displayed after partipation stats.

app/assets/stylesheets/stats.scss

@@ -40,6 +40,10 @@
       content: image-url('stats_woman.png');
     }
 
+    &.total-investments::before {
+      content: image-url('stats_investments.png');
+    }
+
     &.web::before {
       content: image-url('stats_web.png');
     }