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.
Just flagging this as something to discuss as it is not used and would only be introduced when introducing some of the delete_course workflow management we have on edx-solutions/edx-platform. Recommendation: remove this (and anything that listens on it) until we need it....