Prep Plugin for release on WordPress.org

Escape everything that should be escaped.
Add nonce checks where needed.
Sanitize all inputs.
Apply Code style changes across the codebase.
Correct many deprecation notices.
Optimize load order of many filters.

inc/functions/legacy.php

@@ -149,7 +149,7 @@ function wu_print_signup_field($field_slug, $field, $results) {
 
     <script type="text/javascript">
 			document.addEventListener('DOMContentLoaded', function() {
-				var requires = <?php echo json_encode($field['requires']); ?>,
+				var requires = <?php echo wp_json_encode($field['requires']); ?>,
 						target_field = document.getElementById('<?php echo $field_slug; ?>-field');
 
         var display_field = function(target_field, requires, velocity) {