Add a connection-status indicator to the page.
Allow blank chat messages.
Instead of coroutines, use autobahn websockets with callbacks.

Lets me focus the example on async using conventional syntax.
Avoids the distraction of "yield from".
Rename MyServerProtocol -> ChatProtocol
Simplify.
Javascript can autoreconnect after outage.
No need to explicitly bind to '127.0.0.1'.