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.
I wonder if this should just return the field shortcode itself, rather than an empty string, if it's not currently in a form.