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.
In this particular function we could, but not in the following occurences since they are in separate classes. I figured I'd just keep them the same for consistency. I can change them though.