Make the back button a functional link

js/forum/src/utils/History.js

@@ -85,6 +85,17 @@ export default class History {
     m.route(this.getTop().url);
   }
 
+  /**
+   * Get the URL of the previous page.
+   *
+   * @public
+   */
+  backUrl() {
+    const secondTop = this.stack[this.stack.length - 2];
+
+    return secondTop.url;
+  }
+
   /**
    * Go to the first route in the history stack.
    *