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/pusher/less/forum.less

@@ -0,0 +1,9 @@
+.DiscussionList-update {
+  .Button--color(@alert-color, @alert-bg);
+  margin-bottom: 5px;
+
+  .DiscussionPage & {
+    border-radius: 0;
+    margin-bottom: 0;
+  }
+}