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.
You should already have access to %cont-truncated via _mixins.scss in common, so this should work without the additional rule. If not, I would use the existing text-trunctated above it.  thanks, and sorry for the confusion.