Multisite: Change WP_Network id property to an integer.

For consistency and developer sanity.

Props flixos90.
Fixes #37050.

Built from https://develop.svn.wordpress.org/trunk@37870


git-svn-id: http://core.svn.wordpress.org/trunk@37811 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/theme.php

@@ -1521,6 +1521,10 @@ function get_editor_stylesheets() {
  * The {@see 'init'} hook may be too late for some features.
  *
  * @since 2.9.0
+ * @since 3.6.0 The `html5` feature was added
+ * @since 3.9.0 The `html5` feature now also accepts 'gallery' and 'caption'
+ * @since 4.1.0 The `title-tag` feature was added
+ * @since 4.5.0 The `customize-selective-refresh-widgets` feature was added
  *
  * @global array $_wp_theme_features
  *