CLC-3871: Webcasts: use the lecture property from the API instead of pinging Youtube

https://jira.ets.berkeley.edu/jira/browse/CLC-3871
@christianv , apologies if I'm missing something, but if this class is the only usage of Mediacasts::Video & Mediacasts::Video consists entirely of checking the parameters of this call, could we streamline the code by replacing Mediacasts::Video with a Mediacasts::CourseMedia method?