Add Custom Footer HTML (#1315)

* Add Custom Footer HTML

Straight copy from Custom Header HTML

* Move Custom Footer HTML to exactly before `</body>` tag.

* Fix invalid class name

* Append CustomFooterHTML when preparing the view.

* Some consistency in placing the variable

framework/core/src/Frontend/FrontendView.php

@@ -304,7 +304,7 @@ class FrontendView
         $view->jsUrls = $this->buildJsUrls($baseUrl);
 
         $view->head = $this->buildHeadContent();
-        $view->foot = implode("\n", $this->foot);
+        $view->foot = $this->buildFootContent(array_get($forum, 'data.attributes.footerHtml'));
 
         return $view->render();
     }
@@ -380,6 +380,15 @@ class FrontendView
         return $html;
     }
 
+    protected function buildFootContent($customFooterHtml)
+    {
+        $html = implode("\n", $this->foot);
+
+        $html .= $customFooterHtml;
+
+        return $html;
+    }
+
     /**
      * @return CompilerInterface
      */