Webpack (#33)

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

extensions/mentions/js/webpack.config.js

@@ -0,0 +1,3 @@
+const config = require('flarum-webpack-config');
+
+module.exports = config();