Embeds: Introduce embed templates into the template hierarchy via theme-compat.

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.

Built from https://develop.svn.wordpress.org/trunk@36693


git-svn-id: http://core.svn.wordpress.org/trunk@36660 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/template-loader.php

@@ -40,12 +40,13 @@ elseif ( is_trackback() ) :
 	include( ABSPATH . 'wp-trackback.php' );
 	return;
 elseif ( is_embed() ) :
-	$template = ABSPATH . WPINC . '/embed-template.php';
+	$template = ABSPATH . WPINC . '/theme-compat/embed.php';
 
 	/**
 	 * Filter the template used for embedded posts.
 	 *
 	 * @since 4.4.0
+	 * @since 4.5.0 The default template path changed to wp-includes/theme-compat/embed.php
 	 *
 	 * @param string $template Path to the template file.
 	 */