Fix "radio-custom" and "checkbox-custom" usage

[liviubalan]

See "radio_widget" block from https://github.com/symfony/twig-bridge/blob/3.4/Resources/views/Form/bootstrap_4_layout.html.twig

[javiereguiluz]

This looks OK to me, but let's ping some Bootstrap form them expert (@mpiot) to double check it. Thanks!

[mpiot]

Thanks liviubalan :-)
Indeed, the classes: "radio-custom", " radio-inline", "checkbox-custom" or "checkbox-inline" must be on the label, no on the widget.

[javiereguiluz]

Thanks for the review @mpiot and thanks @liviubalan for your contribution. However, I just realized that this was fixed in #9572 so I must close this as "duplicated" and merge the older pull request. Thanks for understanding it!