MDL-49059 medialib: Support links to Youtube videos within playlists

As well as embedding playlists, you can embed specific videos from them -
resulting in the playlist UI displaying, but starting with the specified
video rather than the first in the playlist.  Previously, just the
specified video would embed - without the playlist UI.

lib/medialib.php

@@ -542,6 +542,13 @@ protected function embed_external(moodle_url $url, $name, $width, $height, $opti
             $params .= "start=$start&";
         }
 
+        $listid = $url->param('list');
+        // Check for non-empty but valid playlist ID.
+        if (!empty($listid) && !preg_match('/[^a-zA-Z0-9\-_]/', $listid)) {
+            // This video is part of a playlist, and we want to embed it as such.
+            $params .= "list=$listid&";
+        }
+
         return <<<OET
 <span class="mediaplugin mediaplugin_youtube">
 <iframe title="$info" width="$width" height="$height"