When parsing an `[embed]` shortcode into a TinyMCE view, don't attempt to append any returned `<script>`s to the editor's `<head>`. This affects only a few supported endpoints: Issuu and Twitter.

See #28195.



git-svn-id: https://develop.svn.wordpress.org/trunk@28594 602fd350-edb4-49c9-b593-d223f7449a82