Update presenter notes on change

[pierwill]

Current behavior: When making a change to the presenter notes and viewing in-browser with -p 8000, in order to view the new notes one has to refresh the main presentation tab and open a new presenter console.

Desired behavior: Changes to presenter notes propagate the presenter console for live viewing.

[pierwill]

(I love hovercraft - thanks for your hard work! :) )

[regebro]

I am very slowly working on this on the asyncio branch. It works for the main presentation, but the presenter console closes on refresh, and there are some other issues.