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.
@srpearce just a comment on your suggested text. We'll always allow for enrollment in the course regardless if they had completed the pre-reqs. We won't let them into the course from the dashboard....