Flowerhack/merge ssck and cl

LMS-2858
Actually, I was wondering about LocationBase—this change was the result of s/SlashSeparatedCourseKey/CourseLocator, and I know @sarina encountered some wonkiness when trying to figure out where to pull stuff from `LocationBase` while merging Locations/Locators.

Will LocationBase still even need to exist as a thing?  Should I delete this class?  etc