HTMLForm: Add hide-if

SecurePoll will need a way to display a field only if another field has
a particular value.

We already have this for a limited case in HTMLSelectOrOtherField; this
makes it possible to specify that any particular field should be hidden
based on any other field.

Change-Id: I5d2e6fb1efba0ad97647ac140e2b9a9ac0aee06e