It's very frustrating having pusher installed, but not seeing new posts pop up on non public
discussions. This PR simply re-uses the existing channels per user by reading
all similar private user channels (which require authentication) and then pushing
the websocket message over the line. This only happens in case the discussion is
not public and it only sends to connected users.
See https://github.com/flarum/core/pull/1367
* Replace gulp with webpack and npm scripts for JS compilation
* Set up Travis CI to commit compiled JS
* Restructure `js` directory; only one instance of npm, forum/admin are "submodules"
* Restructure `less` directory
