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.
@caesar2164 what does "get_course_about_section(course, "prerequisites")" match? Does it try and find a "prerequisites" element inside of the course about page text? 

You can see the new setting being added by this PR here: http://invis.io/5P1L1PG4K