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.
As with cms/common.py -- should the app installation be moved into aws.py (ex: https://github.com/edx-solutions/edx-platform/blob/master/lms/envs/aws.py#L489)