Use counter caches to improve spam scoring

Now that we have the counter caches we can take all edits and traces
into account without incurring the cost of having to scan the database
to count them all.

app/models/user.rb

@@ -207,8 +207,8 @@ def delete
   ##
   # return a spam score for a user
   def spam_score
-    changeset_score = self.changesets.limit(10).length * 50
-    trace_score = self.traces.limit(10).length * 50
+    changeset_score = self.changesets.size * 50
+    trace_score = self.traces.size * 50
     diary_entry_score = self.diary_entries.inject(0) { |s,e| s += e.body.spam_score }
     diary_comment_score = self.diary_comments.inject(0) { |s,c| s += c.body.spam_score }