Users: Introduce the concept of a large site to single site installations.

Currently in WordPress multisite there is a concept of large networks. The function `wp_is_large_network` is used to determine if a network has a large number of sites or users. If a network is marked as large, then 
expensive queries to calculate user counts are not run on page load but deferred to scheduled events. However there are a number of places in a single site installation where this functionality would also be useful, as 
expensive calls to count users and roles can make screens in the admin extremely slow.

In this change, the `get_user_count` function and related functionality around it is ported to be available in a single site context. This means that expensive calls to the `count_users` function are replaced with 
calls to `get_user_count`. This change also includes a new function called `wp_is_large_user_count` and a filter of the same name, to mark if a site is large.

Props johnbillion, Spacedmonkey, Mista-Flo, lumpysimon, tharsheblows, obenland, miss_jwo, jrchamp, flixos90, macbookandrew, pento, desrosj, johnjamesjacoby, jb510, davidbaumwald, costdev. 
Fixes #38741.



git-svn-id: https://develop.svn.wordpress.org/trunk@53011 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/includes/class-wp-posts-list-table.php

@@ -1653,7 +1653,7 @@ class WP_Posts_List_Table extends WP_List_Table {
 				<?php endif; // $bulk ?>
 
 				<?php
-				if ( post_type_supports( $screen->post_type, 'author' ) ) {
+				if ( post_type_supports( $screen->post_type, 'author' ) && ! wp_is_large_user_count() ) {
 					$authors_dropdown = '';
 
 					if ( current_user_can( $post_type_object->cap->edit_others_posts ) ) {