Contact Form: Register field shortcode permanently

The activeness of the parsing is toggled via a variable that is set to true whenever the shortcode is needed and false when it is not (and is parsed to empty string). 

Requires a new variable to statically store the form we're currently looking at for decoupling field parsing from the form instance.
Couldn't these just be `self::$using_contact_form_field` rather than spelling out the class?  Or are there child class issues with that?