Add installation progress dialog

[mattpap]

This may require websockets.

[lidavidm]

Python websocket libraries:

• flask-socketio: Works with the popular socket.io library, which provides fallbacks for browsers that don't support WebSockets, but only with old versions of the library and only under Python 2.
• flask-sockets: Only provides plain WebSocket support, still doesn't work with Python 3. Main problem for these two libraries is that gevent isn't supported under Python 3.
• sockjs: Doesn't seem to integrate with Flask directly. Instead, tornado/cyclone would serve the app, delegating to sockjs's Python bindings for a WebSocket connection and to Flask otherwise.

Sockjs seems like the best bet right now, as it works with Python 3.

[mattpap]

Thanks. It would be good to have a 2/3 solution. Usually either the first or the later is supported. I was playing with another option, autobahn, which claims to support both.

[lidavidm]

Both autobahn and Tornado/SockJS look comparable, so I guess the issue is whether we prefer Twisted (the asyncio version doesn't have WSGI integration?) or Tornado for the server.

[tswicegood]

Just giving each a cursory glance, sockjs looks more Pythonic. (on_message vs onMessage). All things being equal, I would lean toward a library that follows the underscore naming just because I would expect less surprise from it.

That said, neither of these libraries have any testing -- so tread lightly with them.