Add new user management features to the WP network admin
Adds a Network Users Manager screen under Users in the network admin, available to users with the manage_network_users capability:
- Browse all network users and all sites from a single place.
- Edit a user's role across every site, or edit every user's role on a given site, from one form.
- Overview of every site and its users.
- Saved user config: save a site's
user → roleassignments as a reusable preset, then apply it to another site's form in one click — only the users in the preset are pre-filled, others are left untouched.