WL-105 Show course cards sorted by start dates, latest first.

@chrisndodge @smagoun Kindly review the changes for WL-105 (sorting order of home page course cards)
Hi Afzal, can you change this to be:

from microsite_configuration import microsite

;
;

then 

if microsite.get_value("ENABLE_COURSE_SORTING_BY_START_DATE", settings.FEATURES["ENABLE_COURSE_SORTING_BY_START_DATE"])

This will allow us to override behavior on a per-white label instance. Thanks