Webpack (#9)

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/akismet/js/src/admin/index.js

@@ -0,0 +1,7 @@
+import app from 'flarum/app';
+
+import AkismetSettingsModal from './components/AkismetSettingsModal';
+
+app.initializers.add('flarum-akismet', () => {
+  app.extensionSettings['flarum-akismet'] = () => app.modal.show(new AkismetSettingsModal());
+});