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.


git-svn-id: https://develop.svn.wordpress.org/trunk@36693 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/theme-compat/embed.php

@@ -0,0 +1,23 @@
+<?php
+/**
+ * Contains the post embed template.
+ *
+ * When a post is embedded in an iframe, this file is used to
+ * create the output.
+ *
+ * @package WordPress
+ * @subpackage oEmbed
+ * @since 4.4.0
+ */
+
+get_header( 'embed' );
+
+if ( have_posts() ) :
+	while ( have_posts() ) : the_post();
+		get_template_part( 'embed', 'content' );
+	endwhile;
+else :
+	get_template_part( 'embed', '404' );
+endif;
+
+get_footer( 'embed' );