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/less/forum.less

@@ -0,0 +1,10 @@
+.Badge--sticky {
+  background: #d13e32;
+}
+.DiscussionStickiedPost {
+  & .EventPost-icon,
+  & .EventPost-info,
+  & .EventPost-info a {
+    color: #d13e32;
+  }
+}