Instructors can edit default response feedback text

Previously, instructors were unable to edit the feedback prompt text ("I noticed that
this response...") displayed in the response box asking a student to provide feedback
on the peer responses they received.

To allow instructors to edit this, I've added an editable xml element (feedback_default_text).
The text within the element defaults to "I noticed that this response...," but an instructor
can change this text to whatever he or she wants by editing the text within the xml element.

Editing of this new element has also been added to the authoring gui.
Oh yes that's definitely a typo, thanks for catching that! 

I tried placing the if/else check within where the text is added to the textarea, but encountered some issues with whitespace being added that led to a pretty ugly display. I'm playing around with it right now looking for a better way, any suggestions are welcome