`setup_network` is a new meta capability that brings more granular control over the permissions to setup a multisite environment. In a non-multisite environment it falls back to `manage_options` while in a multisite it falls back to `manage_network_options`. The introduction of this capability furthermore allows replacing an `is_super_admin()` check.
Props ashokkumar24 for the original patch.
Fixes#39206. See #37616.
Built from https://develop.svn.wordpress.org/trunk@40390
git-svn-id: http://core.svn.wordpress.org/trunk@40297 1a063a9b-81f0-0310-95a4-ce76da25c4cd