MIT CCX: Fix display of CCXs on dashboard

repairs https://openedx.atlassian.net/browse/TNL-2007 and https://openedx.atlassian.net/browse/TNL-2009

Original Commit Messages:

Update ccx dashboard display to match new course display

Fix display of CCX dates on dashboard to use dates from the CCX instead of the related course.

The fix is implemented by providing date- and course-related APIs on the CCX object itself.  These APIs are mimicked from the same APIs available on a course descriptor.

bring the methods for printing start and end datetimes into line with course by adding UTC under the proper circumstances

add test coverage for new APIs defined on the CCX model

fix quality violation