The GA transformer adds the context.course_id property of a tracking
log event to the event transmitted automatically to Segment in the
"label" property, so GA can use it. This adds it also to the
"courserun_key" property, so that regular users of Segment events can
also query it in a more consistently-named property. However, it does
so only if it is a valid CourseKey.
For DENG-997.
Add CourseKey parsing.