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/AlertManager.less

@@ -0,0 +1,12 @@
+.AlertManager {
+  position: fixed;
+  bottom: 20px;
+  left: 20px;
+  z-index: @zindex-alerts;
+
+  .Alert {
+    display: inline-block;
+    margin-top: 20px;
+    .box-shadow(0 2px 6px @shadow-color);
+  }
+}