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.
@marcotuts - I only removed styling that made prerequisites different than the rest of the items in the sidebar... I think you should connect this to the preexisting "prerequisites" conditional. See my other comment.