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.
I would suggest adding a class to the span value here (perhaps .pre-requisite, if we keep the naming consistency with the fields above this one.) 

Additionally, I would set the max-width of this span to be 50%, and @extend %cont-trunctated so that you have long course titles hidden. Especially once this area functions as a link to the relevant course page (upcoming story), the truncated text won't impact understandability of which course to complete.