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/LoadingIndicator.less

@@ -0,0 +1,14 @@
+// ------------------------------------
+// Loading Indicators
+
+.LoadingIndicator {
+  position: relative;
+  color: @muted-color;
+}
+.LoadingIndicator--inline {
+  display: inline-block;
+  width: 25px;
+}
+.LoadingIndicator--block {
+  height: 100px;
+}