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/theme-compat/embed-404.php

@@ -0,0 +1,37 @@
+<?php
+/**
+ * Contains the post embed content template part.
+ *
+ * When a post is embedded in an iframe, this file is used to
+ * create the content template part output if the active theme does not include
+ * a content-embed.php template.
+ *
+ * @package WordPress
+ * @subpackage Theme_Compat
+ * @since 4.5.0
+ */
+?>
+<div class="wp-embed">
+	<p class="wp-embed-heading"><?php _e( 'Oops! That embed can&#8217;t be found.' ); ?></p>
+
+	<div class="wp-embed-excerpt">
+		<p>
+			<?php
+			printf(
+				/* translators: %s: a link to the embedded site */
+				__( 'It looks like nothing was found at this location. Maybe try visiting %s directly?' ),
+				'<strong><a href="' . esc_url( home_url() ) . '">' . esc_html( get_bloginfo( 'name' ) ) . '</a></strong>'
+			);
+			?>
+		</p>
+	</div>
+
+	<?php
+	/** This filter is documented in wp-includes/theme-compat/embed-content.php */
+	do_action( 'embed_content' );
+	?>
+
+	<div class="wp-embed-footer">
+		<?php the_embed_site_title() ?>
+	</div>
+</div>