Webpack (#10)

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/sticky/js/src/admin/index.js

@@ -0,0 +1,13 @@
+import { extend } from 'flarum/extend';
+import app from 'flarum/app';
+import PermissionGrid from 'flarum/components/PermissionGrid';
+
+app.initializers.add('flarum-sticky', () => {
+  extend(PermissionGrid.prototype, 'moderateItems', items => {
+    items.add('sticky', {
+      icon: 'fas fa-thumbtack',
+      label: app.translator.trans('flarum-sticky.admin.permissions.sticky_discussions_label'),
+      permission: 'discussion.sticky'
+    }, 95);
+  });
+});