Fix "required" checkboxes

When you apply the HTML5 "required" attribute to checkboxes, it requires that the user check *all* of the checkboxes instead of a single one of the group. This adds a hack so that only one has to be clicked.

This fixes issue #18.
> Won't this remove the required attribute...

Yeah. But, that's fine. These are only pretty "loosely" required, the checkbox questions also have an "I don't want to answer" requirement. The main reason we're doing "required" is because the UERs don't want people to miss the question by accident.

> Using required on checkboxes isn't supported by the standard...
The problem is that the browser provides nice UI for "required" form fields -- a little dropdown thingy pointing at the missing field. I don't want to use custom UI just for checkboxes but the browser UI for all the other fields. And the browser UI is nicer than any custom UI I'd make.