[CMS] District super admins can assign district users multisite access to individual sites 👤

Super Admins no longer need to grant a "Multisite Management" permission and require each user to add themselves to child sites.
Super admins can now directly assign the child sites the user should have access to.
Assign users different permissions to each site (super admin, website admin, standard, minimal access) and also assign permissions to multiple sites in bulk.
Learn more about the improved Multisite Access feature in the Help Center.