create api endpoints for adding and removing screens

[lonnen]

Video Lego Blocks RFC is a first stab at a fuzzy notion of auto-registering air mozilla components. It's a little long, so I've boiled it down into something more concrete --

Consider a live air mozilla setup -- mozcamp, mozfest, perhaps summit. Cameras turning on and off as different talks, keynotes, and social events start and stop. We have camera blocks that can 'check in' with a predefined url and then start streaming. With the proposed API these camera blocks could register a new CT screen at start up and stream to it, then deregister it when it disconnects. This may require a thin shim, but I think the idea is sound and this would be a necessary first step.