Allowing checkbox labels to contain HTML

The interest is to be able to easily create checkboxes with embedded links, such as "I agree to the Terms of Service" (where 'Terms of Service' is a link that displays the legalese.
The original implementation used textContent, which would convert the embedded html to html entities (&gt;).

Per discussion w Henrik via Gitter, due to security considerations I'm only changing this one instance, though in theory this change could be made in many other cases.