remove dependency on socket.io-client

[briangough]

This PR removes the dependency on socket.io-client and requires the socket.io client files to be served from express in the calling app with a route like this:

app.get('/socket.io/socket.io.js', function (req, res) {
  res.sendFile(
    Path.join(__dirname, 'node_modules/socket.io-client/dist/socket.io.min.js')
  )
})

This removes the unwanted call to uglify and gzip in the upstream file static.js where socket.io was minifying and gzipping files at runtime.

[das7pad]

LGTM. Bonus for the real-time part: Gzip the socket.io.js.min file in CI (e.g. in the Dockerfile or we commit it into the socket.io-client/dist/ folder) and serve it when the client supports compression.