Multisite: Introduce an upgrade_network capability.

Prior to this change, a mix of `is_super_admin()` calls and `manage_network` capability checks was used to determine whether the current user could upgrade the network. With this changeset a dedicated capability is introduced that allows more granular handling.

Props dhanendran for the original patch.
Fixes #39205. See #37616.


git-svn-id: https://develop.svn.wordpress.org/trunk@40404 602fd350-edb4-49c9-b593-d223f7449a82

tests/phpunit/tests/user/capabilities.php

@@ -226,6 +226,7 @@ class Tests_User_Capabilities extends WP_UnitTestCase {
 			'manage_network_themes'  => array(),
 			'manage_network_options' => array(),
 			'delete_site'            => array(),
+			'upgrade_network'        => array(),
 
 			'setup_network'          => array( 'administrator' ),
 			'upload_plugins'         => array( 'administrator' ),
@@ -259,6 +260,7 @@ class Tests_User_Capabilities extends WP_UnitTestCase {
 			'upload_plugins'         => array(),
 			'upload_themes'          => array(),
 			'edit_css'               => array(),
+			'upgrade_network'        => array(),
 
 			'customize'              => array( 'administrator' ),
 			'delete_site'            => array( 'administrator' ),