added edit button and editing form to page title header on unit and cont...

This PR contains Frances' change to the xblock string field editor, adding an edit button and form.

@andy-armstrong @frrrances @cahrens please review
@dan-f This text only makes sense for the container's display name, so it shouldn't live in this template. I guess it should be passed in as a parameter, or we need the text to be very generic.