Reconfigure markup for radio buttons on group admin meta boxes

<label> elements should be applied to the individual radio buttons rather
than the entire set, for better accessibility and standards compliance.

The old top-level label is replaced by a fieldset and legend.

Fixes #5437

Props tw2113

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8013 cdf35c40-ae34-48e0-9cc9-0c9da1808c22