Call preview-thumbnails listeners() function on load (#2604)

The listeners() function in preview-thumbnails.js was never called, so some functionality was broken.

Fixes #2210.

src/js/plugins/preview-thumbnails.js

@@ -123,6 +123,9 @@ class PreviewThumbnails {
       // Check to see if thumb container size was specified manually in CSS
       this.determineContainerAutoSizing();
 
+      // Set up listeners
+      this.listeners();
+
       this.loaded = true;
     });
   };