TinyMCE: improve the previews for embedded WordPress posts:

- Add option to force a sandbox iframe in wpview.
- Use it to show the embedded post preview.
- Remove the deprecated `wpembed` plugin.js

Fixes #39513.
Built from https://develop.svn.wordpress.org/trunk@40019


git-svn-id: http://core.svn.wordpress.org/trunk@39956 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '4.8-alpha-39994';
+$wp_version = '4.8-alpha-40019';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.