Webpack (#14)

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/approval/less/forum.less

@@ -0,0 +1,11 @@
+.Post--unapproved {
+  .Post-header,
+  .Post-body,
+  .EventPost-icon,
+  .EventPost-info {
+    opacity: 0.5;
+  }
+}
+.DiscussionListItem--unapproved {
+  .DiscussionListItem--hidden();
+}