Use websockets to detect tab close

[danvk]

Hopefully fixes #66

Running a websockets server and Python http.server simultaneously proved difficult, so I wound up moving the app entirely over to aiohttp, which includes built-in websocket support on the same port as your http server. This has been reliable so far.

A future extension (mostly useful for debugging) would be to only shut down after all websocket connections have closed, in case you have multiple tabs open.

After going down a bunch of dead ends, I found that signal.raise_signal(signal.SIGINT) was the most effective way to shut down the aiohttp server.

Downsides:

• It's likely that this breaks Windows support.
• aiohttp does pull in some transitive dependencies, but I think they're pretty light.

I'm going to use this locally for maybe a month and see how reliable it is before merging.

Update ~3.5 months later: this is extremely reliable about terminating the process. The one downside I've noticed is that if I want to kill webdiff from the terminal, I have to Ctrl-C it twice.