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.
@caesar2164 - Hi! 12 days ago you merged in a PR that removed the prerequisites section fro m the about pages, possibly because it wasn't used, but I wanted to confirm from you what this refactor/feature was focused on : https://github.com/edx/edx-platform/commit/54b7a578a261abc1438b024d273e753fc5746c13

The team is hoping to add support for soft prerequisites (messaging on course about pages) through this PR, soon to be followed up by gated course prerequisites (which would expose messaging on the LMS dashboard and prevent course access until an author has completed prerequisites. 

Could you describe your changes and any suggestions you have for the team on adding in prerequisites again?  Thanks!