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

https://jira.ets.berkeley.edu/jira/browse/CLC-3871
From ray in chat

> the video part looks like it just relocates 7 lines. If CourseMedia is getting too fat, some other piece might be taken off.The audio code seems inherently different because it's issuing a HTTP request.

I'll try to only move the video part