Add oEmbed support for Mixcloud.

Props Jayjdk.
Fixes #27169.


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Combine `wp_insert_attachment()` and `wp_insert_post()`. `wp_insert_attachment()` becomes a wrapper. Update inline docs.

Props wonderboymusic, DrewAPicture.
See #21963.

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Don't pass embeds through `the_content()` when trying to render MCE previews, leverage `WP_Embed` and `do_shortcode()` instead.

Props kovshenin.
See #28195.


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git-svn-id: http://core.svn.wordpress.org/trunk@28405 1a063a9b-81f0-0310-95a4-ce76da25c4cd
When adding a URL in the `Insert from URL` state in the media modal, attempt to show a preview of the content. Drop the unused width and height fields.

This will probably be iterated upon.

Props helen, jtsternberg, wonderboymusic.
See #15490.

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