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.
@mattdrayer I'm not sure if aws.py is the right place for `FEATURES`. I can not see any `FEATURES` defined in aws.py