Provide common means of hiding fields by default (on first load)

For initially hidden form elements (that appear and then vanish via JavaScript) - we need to use a property in the configuration for the field so that we know the default state and can apply a class. Could perhaps just apply a class in the fields.inc array?

Provide a solution that we can adopt for all such fields, fix any fields that require this, and then add instructions to the design guide.