see #238

This change improves the listing performance: old code had to process all
ApprovalActivity to compute an extension's moderation status and position in
the queue.

Now we maintain a sortkey, a reference to the latest "meaningful" activity
object, and a total comment count. These fields are updated in a post_save
signal.

"Meaningful" activity means moderation status changes:
approved, awaiting changes, awaiting review.

"Non-meaningful" activity shouldn't affect queue position anymore and
extensions without "meaningful" activity should not appear in the queue, but
their respective detail pages should still be reachable via a direct link.
This UX may still need improvement, and #210 may be relevant here.

@@ -56,0 +59,4 @@
+    @property
+    def queue_sortkey(self):
+        timestamp = int(self.date_created.timestamp())
+        return (self.STATUS_CHANGE_TYPES[self.type] << 32) | timestamp