Webpack (#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"
* Refactor JS initializers into Application subclasses
* Maintain partial compatibility API (importing from absolute paths) for extensions
* Remove minification responsibility from PHP asset compiler
* Restructure `less` directory

less/common/Placeholder.less

@@ -0,0 +1,9 @@
+.Placeholder {
+  margin-top: 40px;
+
+  p {
+    font-size: 1.4em;
+    color: @muted-more-color;
+    text-align: center;
+  }
+}