Add prevent deleting sub content of linked directories

Sometimes devs may link to library folders elsewhere, then we
should prevent traversing and deleting the linked content.
Default behavior now is to remove the link and not its content.

h5p.classes.php

@@ -2246,9 +2246,23 @@ class H5PCore {
     if (!is_dir($dir)) {
       return false;
     }
+    if (is_link($dir)) {
+      // Do not traverse and delete linked content, simply unlink.
+      unlink($dir);
+      return;
+    }
     $files = array_diff(scandir($dir), array('.','..'));
     foreach ($files as $file) {
-      (is_dir("$dir/$file")) ? self::deleteFileTree("$dir/$file") : unlink("$dir/$file");
+      $filepath = "$dir/$file";
+      // Note that links may resolve as directories
+      if (!is_dir($filepath) || is_link($filepath)) {
+        // Unlink files and links
+        unlink($filepath);
+      }
+      else {
+        // Traverse subdir and delete files
+        self::deleteFileTree($filepath);
+      }
     }
     return rmdir($dir);
   }