Gracefully handle tab hoarders

[thomaswilburn]

Some people, not to name names, might use the rig in an instance of Chrome that has so many tabs open that they've started to form civilizations amongst themselves. In this case, it'd be easy for there to be multiple tabs open to different graphics in the rig, all of which would then consume resources whenever a change was made in one and they all auto-reload. As such, we should either:

• Detect when other tabs are open using a shared message channel and warn them about it, possibly offering a mechanism to switch to and close those tabs, or
• Find a way to scope our live reload messages so that they're only sent to clients with the current graphic open.