Add accessible fieldset for checkboxes and radio buttons

As per https://www.w3.org/WAI/tutorials/forms/grouping , grouped form controls like radio buttons/checkboxes must be enclosed in <fieldset> and the <legend> element act as a header to identify the group of elements. Currently, when we review such block with WAVE accessibility tool, it complains about:

Proposal

1. Enclose the checkbox/radio button block in a fieldset
2. Add a special class to that fieldset say crm-sr-fieldset which hides it in front-end forms (public and backoffice forms)
3. Use label of checkbox/radio buttons as its legend header.