Add manual livestream controls

[rossng]

There should be a way to detach a livestream from the schedule and start directing it manually. Some scenarios of particular interest:

• An author doesn't turn up to present an item, so the organiser manually plays a prerecorded video instead
• A live presentation ends early, but the Q&A session is separate event. The organiser manually switches to the Q&A event ahead of the expected schedule.
• Someone wants to present a genuinely unscheduled livestream including prerecorded content at times that are not known prior to starting.
• Something goes seriously wrong in the livestream and the broadcast must be terminated.

There is a fair bit of design work to be done here before starting.

Split from #38

[rossng]

Partially resolved by #261