Bootstrap justified button group for radio-btns

A slight variant on the radio-button-group uses Bootstrap's "justified
button group" option to make the button group fit the full width of the
enclosing block.

It requries adding an extra button-group around each button... which
unfortunately breaks Bootstrap's own styling of the button-groups by
adding an extra layer, and thus requires you to add an extra style into your
stylesheet in order to re-hide the original radio-buttons.