Extract method to get the stats key in stats

This way we remove a bit of duplication and it'll be easier to change
the `stats_cache` method.

app/models/concerns/statisticable.rb

@@ -224,4 +224,8 @@ module Statisticable
         I18n.t("stats.age_range", start: start, finish: finish)
       end
     end
+
+    def stats_cache(key, &)
+      Rails.cache.fetch(full_cache_key_for(key), &)
+    end
 end