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.
