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.
@marcotuts - I would try to join the new content with this field/conditional, because otherwise you are making it possible to have multiple "prerequisites" fields, which is weird...