MDL-77015 mod_data: consistent escaping of template field names.

* When creating default templates, avoid escaping the field names
  contained within [[ ]] brackets;
* When managing field, ensure the field names are escaped.

mod/data/classes/template.php

@@ -993,7 +993,7 @@ class template {
                 $errors .= $renderer->notification(get_string(
                     'missingfieldtype',
                     'data',
-                    (object)['name' => $field->field->name]
+                    (object)['name' => s($field->field->name)]
                 ));
             }
         } else {