Calculate entry comment count directly

The MT entry table has a column containing a denormal count of visible
comments.  But because MT uses non-transactional tables, that denormal
count can get out of sync with actual comments.

So we now ignore it, and calculate the comment count directly.  This
should prevent issues like this one occurring in future:

https://github.com/blogs-perl-org/blogs.perl.org/issues/191

Implementing this by adding `LEFT JOIN mt_comment` and `GROUP BY
entry_id` to the main query adds about 250 ms to that query's execution
time.  The separate query used here takes about 50 ms, with no
appreciable increase in observed runtime to generate the output.  Way to
go, MySQL query planner.