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.
Based on feedback recently on a separate PR from @cptvitamin for select field accessibility, I would suggest also adding a <form> around this select field with a screenreader visible <button>. You can see course_navigation.html here for a code sample from that other PR I mentioned: https://github.com/edx/edx-platform/pull/6140/files