Splits wp-includes/embed-template.php, introduced in 4.4, into five new templates that can be individually overridden by themes:
* embed.php
* embed-404.php
* embed-content.php
* header-embed.php
* footer-embed.php
Also introduces a new template tag for outputting the site title, `the_embed_site_title()`.
The five new templates live in theme-compat, allowing for graceful fallbacks should themes prefer not to override any or all of them.
Props swissspidy, imath, ocean90, DrewAPicture.
See #34561.
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