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'd change this to:

You must successfully complete <course> before you enroll in this course.

Will it be possible to list more than one prerequisite? That shouldn't affect the text - I'm just curious.