Fix formwidget IDs for nested fields

This removes [ and ] characters from FormWidget field IDs. This is the same as #474 ([your modified version](https://github.com/octobercms/october/blob/master/modules/backend/classes/WidgetBase.php#L134)) but for FormWidgets.

[Demonstration repo](https://github.com/Flynsarmy/oc-badfileupload-plugin)