Some Slack improvement & fix in settings.json

I've updated Slack hook for a better integration & configuration.

Also, I've added a comma after `videostreams` in `settings.json`, because the json was invalid.
And I've renamed `channel*` in `ned*` to avoid an undefined error when spawing ffmpeg.

One things I'm not sure is about the `settings.port` in `server.js`. From my POV, if we defined a port in the configuration, it should be used in the server and not relying on the global configuration (ie: `process.env.*`). But me let me know if I should remove it from my PR.

Also, I've no idea how to run the test suite..