Multisite: Introduce get_main_site_id().

This function can be used to easily get the main site ID of a given network via the optional `$network_id` parameter, which defaults to the current network. The existing `is_main_site()` now uses the new function internally and now accepts an optional `$network_id` parameter as well.

The main purpose of the new function at this point is to ensure that the `WP_Network::$blog_id` property is always set. Magic getters in the class have been adjusted to auto-fill the property when it is accessed and empty. Furthermore the function encapsulates logic that was previously part of `ms_load_current_site_and_network()` and has been replaced with a call to the function now.

Props spacedmonkey, jeremyfelt, johnjamesjacoby, flixos90.
Fixes #29684.


git-svn-id: https://develop.svn.wordpress.org/trunk@41380 602fd350-edb4-49c9-b593-d223f7449a82

tests/phpunit/tests/multisite/network.php

@@ -583,6 +583,15 @@ class Tests_Multisite_Network extends WP_UnitTestCase {
 
 		$this->assertEquals( $original_count + 1, $result );
 	}
+
+	/**
+	 * @ticket 29684
+	 */
+	public function test_network_blog_id_set() {
+		$network = get_network( self::$different_network_id );
+
+		$this->assertSame( (string) self::$different_site_ids[0], $network->blog_id );
+	}
 }
 
 endif;