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/Select.less

@@ -0,0 +1,20 @@
+.Select {
+  display: inline-block;
+  vertical-align: middle;
+
+  select {
+    display: inline-block;
+    width: auto;
+    -webkit-appearance: none;
+    -moz-appearance: none;
+    padding-right: 30px;
+    cursor: pointer;
+    line-height: 1;
+  }
+}
+.Select-caret {
+  margin-left: -30px;
+  pointer-events: none;
+  color: @control-color;
+  .fa-fw();
+}