add: labels for checkboxes

Associating a checkbox with its label improves accessibility and makes
it more convenient to toggle the checkbox.

For most checkboxes, this adds an `id` for the checkbox and associates
the label with it via the `for` attribute, but for checkboxes in a loop
we can’t use that (multiple checkboxes would have the same `id`), so
there the technique of wrapping the input inside the label is used
instead.

Change-Id: I01b3b8cc57bcdb667bae27f6d88e3c65533a21f5