Don't call wp_count_comments() when we don't need to

[johnbillion]

Comments get counted on every admin screen with wp_count_comments(). We can skip a database call by not counting them if the user doesn't have the edit_posts capability.

---

Side note: With a persistent object cache it's possible to have admin screens with zero database queries with this patch applied. As an exercise you could write a database class that lazy loads MySQL and you'll get an admin screen that doesn't load MySQL. Almost completely pointless, but interesting nonetheless.

Patch

I think 'moderate_comments' would be a more appropriate capability to check. Related: #12104

Related: #11409, #14751