Improve legend tag rendering in forms

The legend html was not escaped and was ignoring block settings coming
from the form editor.
This commit adds a new helper function for rendering legend that respects
applies block settings and also escapes the output.
[MAILPOET-4471]

mailpoet/lib/Form/Block/Checkbox.php

@@ -33,7 +33,7 @@ class Checkbox {
     $fieldValidation = $this->rendererHelper->getInputValidation($block, [], $formId);
 
     $html .= '<fieldset>';
-    $html .= '<legend>' . $block['params']['label'] . '</legend>';
+    $html .= $this->rendererHelper->renderLegend($block, $formSettings);
 
     $options = (!empty($block['params']['values'])
       ? $block['params']['values']