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.
Hmm yeah, that's a good point. I think we should, since that is the default behavior when a shortcode is not defined anyway.