Multisite: Introduce a setup_network capability for setting up multisite.

`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.


git-svn-id: https://develop.svn.wordpress.org/trunk@40390 602fd350-edb4-49c9-b593-d223f7449a82

tests/phpunit/tests/user/capabilities.php

@@ -227,6 +227,7 @@ class Tests_User_Capabilities extends WP_UnitTestCase {
 			'manage_network_options' => array(),
 			'delete_site'            => array(),
 
+			'setup_network'          => array( 'administrator' ),
 			'upload_plugins'         => array( 'administrator' ),
 			'upload_themes'          => array( 'administrator' ),
 			'customize'              => array( 'administrator' ),
@@ -254,6 +255,7 @@ class Tests_User_Capabilities extends WP_UnitTestCase {
 			'manage_network_plugins' => array(),
 			'manage_network_themes'  => array(),
 			'manage_network_options' => array(),
+			'setup_network'          => array(),
 			'upload_plugins'         => array(),
 			'upload_themes'          => array(),
 			'edit_css'               => array(),