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.
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