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.
Relevant bok choy tests are in lms_tests.py and some written by @asadiqbal08 which are specific to cms are in a separate branch.