Ziafazal/add pre requisite course via milestones app

@mattdrayer added pre-requisite feature via milestones app. It would show a drop down for course authors to select from list available courses as a pre-requisite course in "Settings & Details" page. Inside lms it would show the appropriate messaging under side bar of course about page.
I'm not sure? The code seems to imply get_course_about_section() returns pre formatted HTML, but I can't seem to backtrack "prerequisites"...