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've been learning a bit more about bok choy -- the general approach is to locate these selector queries (either css or xpath) in the Page object, and then expose methods for the Test object to invoke.

So in this particular case we should move settings_detail.q() into SettingsPage, expose via a method (say, get_prerequisite_course_field), and then self.assertTrue(prerequisite_course_field.present) within test_page_has_prerequisite_field