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)
I have removed the metadata parameter, but the factory puts a default start date of 2030-01-01. Specifying the start explicitly to None in metadata breaks this line : https://github.com/edx/edx-platform/blob/afzaledx/WL-105/common/lib/xmodule/xmodule/course_module.py#L926

Seemingly the start should never be None. This is in line with the Studio's behavior where we're not allowed to skip the start date. Since the default start date is 2030, it's always showing up at the very last for both announcement based and start based sorting.