Eliminate 'prepare broadcasts' step from the streaming pipeline

[rossng]

The 'prepare broadcasts' step has proven to be confusing and problematic. We should eliminate it and use live information from the events and content groups to feed MediaLive.

[rossng]

As part of this, it will be necessary to rewrite the code that syncs the schedule up to MediaLive. At the moment, this code may take the channel down mid-broadcast if the schedule changes in the wrong way.