This allows us to get started with converting all Flarum JavaScript code to TypeScript.
In addition, we will have time to experiment to find the best Webpack configuration before integrating into flarum-webpack-config.
See flarum/flarum-webpack-config#3.
* 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"
* Refactor JS initializers into Application subclasses
* Maintain partial compatibility API (importing from absolute paths) for extensions
* Remove minification responsibility from PHP asset compiler
* Restructure `less` directory
