Convert AJAX to POST request

This AJAX request sends form data in $_GET request query. This is problematic and unreliable when validating superrrr loooooong conditions, especially in environments that use reverse-proxy.

Been having this problem and this PR solves it. :)

data/web/js/site/mailbox.js

@@ -179,9 +179,8 @@ $(document).ready(function() {
     // Get script_data textarea content from form the button was clicked in
     var script = $('textarea[name="script_data"]', $(this).parents('form:first')).val();
     $.ajax({
-      dataType: 'json',
       url: "/inc/ajax/sieve_validation.php",
-      type: "get",
+      type: "post",
       data: { script: script },
       complete: function(data) {
         var response = (data.responseText);