TinyMCE: better way to preview embedded posts (wpembed)

[azaozz]

Currently we load a script in the editor iframe to support previews of embedded posts from other WordPress sites. Better way to do that would be to "sandbox" these previews and load wp-embed.js in the sandbox iframe.

[azaozz]

In 39513.patch:

• Show previews of embedded posts in a sandbox iframe.
• Add support for forcing a sandbox when rendering a wpview.
• Fix rare race condition when inserting the sandbox iframe into the wpview container.
• Retire the wpembed TinyMCE plugin as it is not needed any more. wp-embed.js is now loaded in the sandbox iframe, not appended to the editor iframe.

[azaozz]

In 40019:

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.

[ocean90]

In 40879:

Upgrade: Add wp-includes/js/tinymce/plugins/wpembed to $_old_files.

See #39513.
Fixes #40942.

[ocean90]

In 40880:

Upgrade: Add wp-includes/js/tinymce/plugins/wpembed to $_old_files.

Merge of [40879] to the 4.8 branch.

See #39513, #40942.